[Federal Register Volume 63, Number 22 (Tuesday, February 3, 1998)]
[Notices]
[Pages 5687-5712]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 97-33937]
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DEPARTMENT OF LABOR
Mine Safety and Health Administration
Coal Mine Respirable Dust Standard Noncompliance Determinations
Correction and Republication
Note: For the convenience of the user, notice document 97-33937
is being reprinted in its entirety because of numerous errors in the
document originally appearing at 62 FR 68395-68420, December 31,
1997. Those wishing to see a listing of corrections, please call
Patricia Silvey, Mine Safety and Health Administration, 703-235-
1910.
AGENCY: Mine Safety and Health Administration, Labor.
ACTION: Notice; final policy.
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SUMMARY: This notice announces the Mine Safety and Health
Administration's (MSHA) final policy concerning the use of single,
full-shift respirable dust measurements to determine noncompliance and
issue citations, based on samples collected by MSHA, when the
applicable respirable dust standard is exceeded. This notice should be
read in conjunction with the notice published elsewhere in today's
Federal Register jointly by the Department of Labor and the Department
of Health and Human Services.
EFFECTIVE DATE: This policy is effective March 2, 1998.
FOR FURTHER INFORMATION CONTACT: Ronald Schell, Chief, Division of
Health, Coal Mine Safety and Health; MSHA; 703-235-1358.
SUPPLEMENTARY INFORMATION:
I. About This Notice
This notice provides information about MSHA's new enforcement
policy for the use of single, full-shift respirable dust measurements
obtained by inspectors to determine noncompliance with the respirable
dust standard (applicable standard) under the MSHA coal mine respirable
dust program. A question and answer format has been used to explain the
background for the enforcement policy, the reasons for the policy
change, and the specific elements of the new policy. In addition,
several appendices are attached to and incorporated with this final
notice which address technical issues concerning the new enforcement
policy.
II. Background Information
A. How Has MSHA Sampled Coal Mines for Noncompliance in the Past?
Prior to October 1975, noncompliance determinations were based on
the average of full-shift measurements collected from individual
occupations on multiple shifts. MSHA interprets a full shift for
underground coal mines to mean the entire shift worked or 8 hours in
duration or whichever time period is less (30 CFR 70.201(b)). The need
to reduce the Agency's administrative burden attributable to inspector
sampling prompted MSHA to revise its underground health inspection
procedures and redirect the Agency's enforcement resources away from
sampling and toward assessing the effectiveness of mine operators'
respirable dust control programs.
Since October 1975, MSHA has determined noncompliance with the
applicable standard based on the average of measurements obtained for
different occupations during the same shift of a mechanized mining unit
(MMU), or on the average of measurements obtained for the same
occupation on successive days. The term MMU is defined in 30 CFR
70.2(h) to mean a unit of mining equipment, including hand loading
equipment, used for the production of material. MSHA inspectors
routinely sample multiple occupations to determine compliance with the
applicable standard, assess the effectiveness of mine operators' dust
control programs, determine whether excessive levels of quartz dust are
present, and verify the designation of the ``high risk occupation''
(now referred to as the ``designated occupation'' or ``D.O.''--the
occupation on a working section exposed to the highest respirable dust
concentration) to be sampled by mine operators.
Under the sampling procedures in place between 1975 and 1991, MSHA
inspectors would collect full-shift measurements from the working
environment of the ``D.O.'' and four other occupations, if available,
on the first day of sampling each MMU. The mine operator was cited if
the average of all measurements obtained during the same shift exceeded
the applicable standard by at least 0.1 milligram of respirable dust
per cubic meter of air (mg/m3). If one or more measurements
exceeded the applicable standard but the average did not, the Agency's
practice was to continue sampling for up to four additional production
shifts or days. If the inspector continued sampling after the first day
because a previous measurement exceeded the applicable standard,
noncompliance determinations were based on either the average of all
measurements taken or on the average of measurements taken on any one
occupation. Thus, if the average of measurements taken over more than
one day on all occupations was less than or equal to the applicable
standard, but the average of measurements taken on any one occupation
exceeded the value set by MSHA (based on the cumulative concentration
for two or more measurements exceeding 10.4 mg/m3, which is
equivalent to a 5-measurement average exceeding 2.0 mg/m3),
the operator was cited for exceeding the applicable standard.
In some instances, MSHA inspectors sampled for a maximum of five
production shifts or days before making a noncompliance determination.
However, most citations issued prior to 1991 were based on the average
of multiple measurements on different occupations collected during a
single shift. To illustrate, MSHA conducted a computer simulation using
data from 3,600 MMU inspections conducted between October 1989 and June
1991. This simulation showed that a total of 293 MMUs would have met
the criteria to be found in noncompliance with the applicable standard
based solely on the average of multiple measurements. Two hundred
forty-two of those noncompliance determinations, or 83 percent, met the
citation criteria based on sampling results from the first day of MSHA
sampling, rather than from multi-day sampling. Only 51 MMUs, or 17
percent, were citable based on the average of measurements collected
over multiple shifts or days. These statistics clearly show that the
citation criteria were met based not only on the average of
measurements taken during several shifts, but also on the average of
[[Page 5688]]
multiple measurements obtained during the same shift.
B. Why Did MSHA Establish the Coal Mine Respirable Dust Task Group and
Initiate the Spot Inspection Program?
In 1991 concerns were raised about the adequacy of MSHA's program
to control respirable coal mine dust in underground coal mines. In
response to these issues, MSHA established the Coal Mine Respirable
Dust Task Group (Task Group) to comprehensively evaluate the
effectiveness of the Agency's respirable dust program.
The Task Group was directed to consider all aspects of the current
program, including the role of the individual miner in the sampling
program; the feasibility of MSHA conducting all sampling; and the
development of new and improved monitoring technology, including
technology to continuously monitor the mine environment. Among the
issues addressed by the Task Group was the actual dust concentration to
which miners are exposed. As a result, the Agency initiated a special
respirable dust ``spot inspection program'' (SIP), designed to provide
the Agency with more accurate information on the dust levels to which
miners were exposed, through sampling, in the underground coal mine
environment.
C. How Was Sampling Accomplished During the SIP?
Because of the large number of mines and MMUs involved and the need
to obtain data within a short time frame, sampling during the SIP was
limited to a single shift or day, a departure from MSHA's normal
sampling procedures. As a result, the Agency determined that if the
average of multiple occupation measurements taken on an MMU during any
one-day inspection did not exceed the applicable standard, the
inspector would review the result of each sample individually. If any
individual measurement exceeded the applicable standard by an amount
specified by MSHA, a citation would be issued for noncompliance,
requiring the mine operator to take immediate corrective action to
lower the average dust concentration.
The sampling practice under the SIP was similar to the practice of
the Metal/Nonmetal Health Division of MSHA, and the Occupational Safety
and Health Administration (OSHA), which use a single, full-shift
measurement for noncompliance determinations, and provides for a margin
of error to account for uncertainty in the measurement process
(sampling and analytical error). This resulted in the issuance of
citations using a single, full-shift measurement only when there was a
high level of confidence that the applicable standard was actually
exceeded.
Thus, during the SIP inspections, MSHA inspectors cited violations
of the current 2.0 mg/m\3\ standard if either the average of five
measurements taken on a single shift was greater than or equal to 2.1
mg/m\3\, or any single, full-shift measurement was greater than or
equal to 2.5 mg/m\3\. Similar adjustments were made when the 2.0 mg/
m\3\ standard was reduced due to the presence of quartz (crystalline
silica) dust in the mine environment.
D. What Did the SIP Show About MSHA's Sampling Policy?
MSHA's review of the SIP inspections showed that 28 percent of 718
MMUs sampled exceeded the applicable standard and would have been
citable based on a single, full-shift measurement, but only 12 percent
would have been citable using the average of all measurements for the
MMU.
Based on the data from the SIP inspections, the Task Group
concluded that the Agency practice of determining noncompliance based
solely on the average of multiple measurements did not always reveal
situations in which miners were overexposed. For example, if the
measurements obtained for five different occupations within the same
MMU were 4.1, 1.0, 1.0, 2.5, and 1.4 mg/m\3\, the average concentration
would be 2.0 mg/m\3\ and no enforcement action would be taken, even
though the dust measurements for two of these occupations significantly
exceeded the applicable standard. While such individual measurements
were not cited prior to the SIP, they would clearly demonstrate that
some miners were overexposed. MSHA policy prior to the SIP however,
required the inspector to return to the mine on the next production day
and resume sampling, rather than issue a citation at the time the
overexposures were discovered.
E. Why Did MSHA Decide To Permanently Adopt the SIP Procedures?
The SIP inspections revealed instances of overexposure that were
masked by the averaging of results across different occupations. This
showed that miners would not be adequately protected if noncompliance
determinations were based solely on the average of multiple
measurements. The process of averaging dilutes a high measurement made
at one location with lower measurements made elsewhere. Similarly,
averaging a number of full-shift measurements can obscure cases of
overexposure.
Additionally, the Task Group recognized that the initial full-shift
samples collected by an inspector are likely to show higher dust
concentrations than succeeding samples collected on subsequent shifts
during the same inspection. MSHA's data showed that the average
concentration of all samples taken on the same occupation on the first
day of an inspection was almost twice as high as the average
concentration of those taken on the second day. MSHA recognized that
sampling on successive days after an inspector first appears could
result in measurements that are not representative of dust conditions
to which miners are typically exposed. Unrepresentative measurements
would arise if mine operators anticipated the continuation of inspector
sampling and made adjustments in dust control parameters or production
rates to reduce dust levels during the subsequent monitoring. None of
this is specifically prohibited by MSHA regulations. As a result of
these findings, which indicated that miners were at risk of being
overexposed, MSHA decided to permanently adopt use of the single, full-
shift measurement inspection policy initiated during the SIP. These
procedures were used by MSHA until the issuance of the decision by the
Federal Mine Safety and Health Review Commission in the case of
Keystone Coal v. Sec. of Labor, 16 FMSHRC 6 (Jan. 4, 1994). Since that
decision, MSHA has reverted to its previous practice of basing
noncompliance determinations on the average of multiple, full-shift
measurements. (Please see the notice of joint finding by the Secretary
of Labor and the Secretary of Health and Human Services (HHS) published
elsewhere in today's Federal Register for an explanation of this
decision.)
III. Why MSHA Is Revising Its Enforcement Policy
A. What Has Changed To Warrant Revising the Existing Enforcement
Policy?
During the public hearings held on the proposed joint finding that
a single, full-shift sample is an accurate measurement, during the
public meetings held on this enforcement policy notice, and in other
comments submitted to the Agency, several commenters questioned why the
current program should be altered. The commenters asserted that MSHA's
practice of issuing citations based on the average of multiple
measurements has
[[Page 5689]]
been in effect since the 1970s, that technology and equipment
associated with sampling remain essentially the same, and that
substantial progress had been made in lowering respirable dust levels
at U.S. coal mines.
As stated in the final notice of joint finding published elsewhere
in today's Federal Register, significant improvements have in fact been
made in the dust sampling process. Although MSHA agrees that progress
has been made in reducing average dust concentrations, the SIP
inspections clearly showed instances of excessive dust concentrations
that would have been masked by the procedure of averaging measurements.
Specifically, of the 718 SIP MMUs with valid single, full-shift
measurements, 203 MMUs had at least one single, full-shift measurement
that was citable, while only 88 MMUs met or exceeded the citation
threshold based on the average of multiple measurements. This clearly
shows that under the procedure of averaging measurements miners would
be at risk of being overexposed and MSHA would be unable to require
operators to take corrective actions to protect them.
MSHA believes that a single, full-shift measurement is more likely
to detect excessive dust concentrations and thus protect miners than a
measurement average across multiple occupations on a single shift or
across multiple shifts for a single occupation. MSHA's computer
simulation which analyzed data from over 3600 MMU inspections conducted
between October 1989 and June 1991, showed that 814 MMUs had citable
overexposures based on individual samples, but only 298 of these
overexposures were citable on the average of measurements made within
the MMU. Subsequent to the SIP, between January 1992 and December 1993,
MSHA continued making noncompliance determinations on a single, full-
shift measurement, and 74 percent or 488 of the 658 MMUs cited by
inspectors as having overexposures were found to be out of compliance
based on a single, full-shift measurement, requiring mine operators to
take appropriate corrective action. This experience clearly
demonstrates that citing on a single, full-shift measurement, as
opposed to citing on the average of measurements taken over multiple
shifts, impacts miners directly, because it requires mine operators to
take more prompt corrective action once an overexposure has been
identified. This reduces the risk to miners of continued exposure to
dust concentrations above the applicable standard on subsequent shifts.
Furthermore, both NIOSH, in its recently issued criteria document,
and the Secretary of Labor's Advisory Committee on the Elimination of
Pneumoconiosis Among Coal Mine Workers recommended the use of single,
full-shift measurements for determining compliance. According to the
Committee report, issued in October 1996, the MSHA practice of not
issuing citations based on single, full-shift samples ``is not
protective of miner health, moreover, it is inconsistent with the
stated intent of the Coal Act and the Mine Act, which require that
exposure be at or below the exposure limit for each shift.''
B. Why Will MSHA No Longer Rely On Averaged Measurements of Dust
Concentrations To Determine Noncompliance?
MSHA's current enforcement strategy does not provide the optimal
level of possible health protection. Basing noncompliance
determinations on the average of different occupational measurements
dilutes a measurement of high dust exposure with a lower measurement
made at a different occupational location. Likewise, averaging
measurements obtained for the same occupation over different shifts
does not ensure that the concentration of respirable dust is maintained
at or below the applicable standard during each shift. Section
202(b)(2) of the Mine Act clearly requires that dust concentrations be
maintained at or below the applicable standard ``* * * during each
shift to which each miner in the active workings'' is exposed.
Some commenters proposed that MSHA continue to average at least
five separate measurements prior to making a noncompliance
determination. They stated that abandoning this practice would reduce
the accuracy of noncompliance determinations. Specifically, these
commenters maintain that the average of dust measurements obtained at
the same occupational location on different shifts more accurately
represents dust exposure to a miner than a single, full-shift
measurement. These commenters favored the retention of existing MSHA
policy on the grounds that not averaging measurement results would
reduce accuracy to unacceptable levels. Other commenters agreed with
MSHA that the averaging of multiple samples dilutes measurements of
dust concentration and masks specific instances of overexposure. Some
of these commenters stated that averaging distorts not only the
estimate of dust concentration applicable to individual shifts, but
also biases the estimate of exposure levels over a longer term.
According to these commenters, this is because dust control measures
and work practices affecting dust concentrations are frequently
modified in response to the presence of an MSHA inspector over more
than a single shift. These commenters argued that the presence of the
MSHA inspector causes the mine operator to be more attentive to dust
control than normal.
Section 202(b) of the Mine Act requires each mine operator to
``continuously maintain the average concentration of respirable dust in
the mine atmosphere during each shift to which each miner is exposed''
at or below the applicable standard. The greater the variation in
mining conditions from shift to shift, the less likely it is that a
multi-shift average will reflect the average dust concentration on any
individual shift. For example, during one shift, production may be high
and dust concentrations may also be correspondingly high. However, the
next shift may experience lower production levels because of equipment
breakdowns or because of unusual mining conditions. In addition, when a
mine operator knows that the MSHA inspector is present, more attention
may be given to ensuring that dust control measures operate
effectively, and this may also affect the concentrations of respirable
coal mine dust found on that shift. Because of such factors, multi-
shift averaging does not improve the accuracy of a noncompliance
determination for the sampled shift. Therefore, MSHA is discontinuing
its policy of relying on averaged dust concentrations. A more technical
discussion of how averaging measurements affects accuracy is given in
Appendix A.
C. Why Has MSHA Decided To Base Noncompliance Determinations Solely on
a Single, Full-Shift Measurement?
One commenter suggested that the new enforcement strategy proposed
in MSHA's February 1994 notice, involving noncompliance determinations
based on either a single sample or on the average of multiple samples,
placed operators in ``double jeopardy'' of being cited--that is, it
provided for two separate evaluations of whether the applicable
standard has been exceeded. This commenter pointed out that this
enforcement strategy would reduce the confidence level at which a
noncompliance determination could be made.
Under the MSHA policy proposed in the February 1994 notice,
measurements made by an MSHA inspector for
[[Page 5690]]
different occupational locations would have been averaged together, not
in order to estimate a hypothetical average concentration, but rather
to ascertain whether dust concentration was excessive at any of the
sampled locations. If the average of measurements across sampling
locations exceeded the applicable standard, then at least one of the
sampling locations would almost certainly have been out of compliance
on the sampled shift. Therefore, the commenter was correct in asserting
that noncompliance at each sampling location would have been evaluated
twice: once using the single measurement specific to that location;
and, if that test did not result in a citation, once again using the
average of all available measurements.
MSHA had determined that this strategy was necessary to provide the
level of health protection to miners required by the Mine Act, and
included this strategy in the proposed policy notice to protect against
cases of evident noncompliance that would otherwise go uncited. For
example, if five occupational measurements of 2.08, 2.28, 2.31, 2.25,
and 2.17 mg/m3 were obtained for an MMU on a 2.0 mg/
m3 standard, no enforcement action would be taken if
noncompliance is determined solely based on a single, full-shift
measurement because no individual measurement meets or exceeds the
Citation Threshold Value (CTV), defined in section IV.B. of this
notice. On the other hand, averaging the measurements results in an
average concentration of 2.22 mg/m3, indicating, with high
confidence, that the applicable standard was exceeded.
Although MSHA originally proposed using a combination of both
strategies for determining noncompliance, various bodies of data show
that such hypothetical occurrences are extremely improbable in
practice. For example, MSHA's computer simulation discussed earlier in
this notice showed that, between October 1, 1989, and June 30, 1991,
298 MMUs would have been found in noncompliance with the applicable
standard based on averaging multiple measurements. All 298 MMUs would
also have been found in noncompliance using the single, full-shift
measurement citation criteria. According to the data from the SIP, only
one noncompliance determination would have been missed if all averaging
had been discontinued. Similarly, analysis of more recent inspector
sampling data for 1995 indicates that miners' health will not be
compromised by discontinuing all measurement averaging. In fact, only
one additional case of noncompliance would have been identified using
averaging in addition to citing on a single, full-shift measurement.
Therefore, MSHA will not continue to use this combination of
strategies.
As explained in the final notice of joint finding published
elsewhere in today's Federal Register, MSHA's improved sampling and
analytical method performs in accordance with the NIOSH Accuracy
Criterion whenever a single, full-shift measurement is at or above 0.36
mg/m3. The Agency believes that, in accordance with section
202(f) of the Mine Act, this enables MSHA to base noncompliance
determinations on a single, full-shift measurement whenever that
measurement is at or above 0.36 mg/m3.
IV. The New Enforcement Policy
A. What Is MSHA's New Enforcement Policy?
MSHA will continue its current dust sampling program as it relates
to where and how many samples an inspector collects during a sampling
shift. Specifically, MSHA will continue to collect multiple
occupational samples for each MMU. The criterion for making
noncompliance determinations has been revised and, under the new
enforcement policy, MSHA will use a control filter capsule to adjust
the resulting weight gain obtained on each exposed filter capsule.
Noncompliance determinations will be based solely on the results of
individual, full-shift samples, and MSHA will issue a citation whenever
noncompliance is demonstrated at a high confidence level. The Agency
will no longer rely on multi-locational or multi-shift averaging of
measurements to determine noncompliance.
The process by which a violation of the applicable standard will be
abated by a mine operator will also remain unchanged. MSHA will
consider a violation to be abated when samples collected in accordance
with 30 CFR 70.201(d) demonstrate that the average dust concentration
in the working environment of the cited occupation is at or below the
applicable standard.
When a measurement exceeds the applicable standard but is less than
the CTV, noncompliance is not demonstrated at a sufficiently high
confidence level to warrant a citation. However, MSHA will consider
whether to target the MMU or environment for additional dust sampling.
See Appendix B for further discussion of why MSHA believes that such
measurements indicate probable overexposure.
B. When Will MSHA Issue a Citation for a Violation of the Applicable
Standard?
MSHA will issue a citation for noncompliance when a single, full-
shift measurement demonstrates, at a high level of confidence, that the
applicable standard has been exceeded. Although MSHA will continue to
collect multiple occupational samples for each MMU, the Agency will
generally issue only one citation for exceeding the applicable standard
on a single shift on any one MMU. However, additional citations may be
issued when excessive dust concentrations are detected for occupations
exposed to different dust generating sources.
To ensure that citations are issued only when there is a high level
of confidence that the applicable standard has been exceeded, MSHA has
developed the Citation Threshold Values (CTV) below. Each CTV listed is
calculated so that citations are issued only when the single, full-
shift measurement demonstrates noncompliance with at least 95 percent
confidence. Citing in accordance with the CTV table does not constitute
a raising of the applicable standard. Instead, it reflects the need for
MSHA to ensure a sufficiently high level of confidence in its
noncompliance determinations. Mine operators are still required to
implement appropriate controls that will maintain the average
concentration of respirable dust at or below the applicable standard on
all shifts.
Citation Threshold Values (CTV) for Citing Violations Based on Single,
Full-Shift Measurements
------------------------------------------------------------------------
Applicable standard (mg/m3) CTV (mg/m3)
------------------------------------------------------------------------
2.0................................................. 2.33
1.9................................................. 2.22
1.8................................................. 2.11
1.7................................................. 2.00
1.6................................................. 1.90
1.5................................................. 1.79
1.4................................................. 1.68
1.3................................................. 1.58
1.2................................................. 1.47
1.1................................................. 1.36
1.0................................................. 1.26
0.9................................................. 1.15
0.8................................................. 1.05
0.7................................................. 0.94
0.6................................................. 0.84
0.5................................................. 0.74
0.4................................................. 0.64
0.3................................................. 0.53
0.2................................................. 0.43
------------------------------------------------------------------------
C. How Will the CTV Table Be Applied?
Each single, full-shift measurement used to determine noncompliance
will
[[Page 5691]]
be the MRE-equivalent dust concentration as calculated and recorded
under MSHA's dust data processing system. Every valid measurement will
be compared with the CTV corresponding to the applicable standard in
effect. If any measurement meets or exceeds that value, a citation will
be issued. However, no more than one citation will be issued based on
single, full-shift measurements from the same MMU, unless separate
citations are warranted for occupations exposed to different dust
generating sources. Therefore, when single, full-shift measurements
from two or more occupations show dust concentrations in violation of
the applicable standard, as illustrated in the examples below, the
inspector will determine the dust generation sources and require the
operator to sample the environment of the occupation most affected by
these sources which is consistent with current practice. In most cases,
this will be the working environment of the ``D.O.'' However, if
noncompliance is indicated based on measurements from two or more
occupations on the same MMU which are exposed to the same dust
generating sources, and which do not involve the ``D.O.,'' the
occupation with the highest dust concentration will be identified in
the citation as the affected working environment. In any case, when an
inspector issues a citation for violation of the applicable standard
under the new policy, the citation narrative will identify the specific
environment or occupation to be sampled by the operator, as well as any
other occupation(s) that exceeded the CTV.
Several commenters requested that the application of the CTV table
be clarified. The following examples illustrate how inspectors will
apply the CTV table and make noncompliance determinations. Suppose that
a measurement of 2.41 mg/m3 is obtained for the ``D.O.'',
and measurements of 2.34, 1.54, and 1.26 mg/m3, are obtained
for three other occupations exposed to the same dust generating sources
as the ``D.O.'' during a single shift on an MMU required to comply with
an applicable standard of 2.0 mg/m3. Because at least one of
the measurements exceeds the 2.33-mg/m3 CTV (the citation
value when the applicable standard is 2.0 mg/m3), a citation
will be issued for exceeding the applicable standard on the shift
sampled. Even though two individual measurements (2.41 and 2.34 mg/
m3) exceeded the CTV, one of which is on the ``D.O.,'' only
one citation will be issued, specifying the ``D.O.'' as the affected
working environment because all occupations were exposed to the same
dust generating sources.
Suppose now that in the previous example the 2.34-mg/m3
measurement was obtained for a roof bolter, and the MMU was ventilated
using a double-split ventilation system. This means that the roof
bolter, working on a separate split of air from that of the continuous
miner, is exposed to a different dust generating source than the
``D.O.'' and, therefore, may not be adequately protected by dust
controls implemented for the ``D.O.'' Consequently, two citations would
be issued.
As another example, consider an MMU with measurements of 2.14,
1.92, 1.82, 1.25, and 1.12 mg/m3. Although none of these
measurements meet the CTV, there is reason to believe that the MMU is
out of compliance, since one of the measurements exceeds the applicable
standard. However, because there is a small chance that the measurement
exceeded the applicable standard because of measurement error, a
citation would not be issued. As discussed elsewhere in this notice,
additional samples would be necessary to verify the adequacy of the
control measures under current operating conditions. Therefore, MSHA
would select this MMU for additional sampling. As discussed in Appendix
B, even if the first measurement were 1.90 mg/m3 instead of
2.14 mg/m3, because of measurement error this would not
demonstrate that the mine atmosphere sampled was in compliance. To
confirm that control measures are adequate, MSHA would need to take
additional samples.
D. What Is the Potential for a Citation To Be Issued Due To Measurement
Error?
Some commenters expressed concern that noncompliance determinations
based on single, full-shift measurements would result in an
unacceptable number of erroneous citations due to measurement error.
These commenters expected that MSHA's new enforcement policy would
result in numerous erroneous citations.
Based on the analysis in Appendix C, MSHA has concluded that,
because of the large ``margin of error'' separating each CTV from the
corresponding applicable standard, use of the CTV table provides ample
protection against erroneous citations. For exceptionally well-
controlled environments (e.g., Case 2 of Appendix C), the probability
that any given citation is erroneous will be substantially less than 5
percent. This probability is even smaller in environments which are not
well controlled (e.g., Case 3 of Appendix C). Therefore, any citation
issued in accordance with the CTV table will be much more likely the
result of excessive dust concentration rather than measurement error.
E. What Will Happen When the Evidence Is Insufficient To Warrant a
Citation?
If the appropriate CTV is not met or exceeded, MSHA will not issue
a citation. As discussed earlier, this does not mean that the sampled
environment is necessarily in compliance. Although in certain cases
there may be insufficient evidence to demonstrate noncompliance, the
measurement may nonetheless indicate a possible overexposure. MSHA
intends to focus on cases of measurements above the applicable standard
but below the CTV, with special emphasis being directed to working
environments required to comply with applicable standards below 2.0 mg/
m3.
If follow-up measurements do not warrant a citation but suggest
that the dust control measures in use may be inadequate, MSHA may
initiate a thorough review of the dust control parameters stipulated in
the mine operator's approved ventilation or respirable dust control
plan to determine whether the parameters should be upgraded.
V. Consequences of the Use of the CTVs in Conjunction With the
Joint MSHA/NIOSH Finding
A. What is the Impact of MSHA's New Enforcement Strategy As Applied
Under the MSHA/NIOSH Joint Finding?
The Agency believes that the application of the CTVs in conjunction
with the MSHA/NIOSH joint notice of finding published elsewhere in
today's Federal Register to single, full-shift samples collected by
MSHA inspectors provides for more efficient detection of noncompliance
by identifying and requiring abatement of individual instances of
overexposure which meet the CTVs. While this issue is more
appropriately addressed in the MSHA/NIOSH joint notice, the rationale
for this conclusion bears repeating here.
The Mine Act is clear in its intent that no miner should be exposed
to respirable coal mine dust in excess of the applicable standard on
any shift. The effect of the joint finding and the new enforcement
strategy set forth here creates incentives for mine operators to
control dust exposure on a continuing basis to minimize the chance of
being found in noncompliance during any MSHA sampling inspection. To
prevent the possibility of any inspector single,
[[Page 5692]]
full-shift measurement exceeding the CTV and resulting in a violation,
mine operators will be more likely to keep dust concentrations at or
below the applicable standard, thereby providing better protection to
miners from overexposures. This becomes evident upon closer examination
of the inspector sampling data from the period when noncompliance
determinations were based on single, full-shift measurements.
MSHA reviewed inspector MMU sampling results for FY 1992, the first
full year during which noncompliance determinations were based on
single, full-shift measurements, and FY 1993, the last year that the
Agency issued citations based on single, full-shift measurements. This
review showed a decline in the number of ``D.O.'' and nondesignated
occupation samples exceeding 2.0 mg/m3, from 16 percent and
10 percent in FY 1992 to 13 percent and 7 percent, respectively, in FY
1993, suggesting that operators were better able to maintain dust
concentrations below the applicable standard. MSHA also conducted a
computer simulation using these data which showed that one of every
four MMU sampling days in FY 1992 would have been found in
noncompliance based on a single, full-shift measurement, compared to
one in five MMU sampling days in FY 1993.
Under the previous enforcement strategy, which utilized averaging,
inspectors cited violations of the applicable standard on the average
of multiple measurements taken on a single shift or on different shifts
or days. Consequently, dust concentrations could be excessive for some
occupations or work locations, but corrective action would not be
required so long as the average of the measurements did not exceed the
applicable standard. For example, averaging occupational measurements
of 3.2, 2.4, 1.5, 1.3 and 1.0 mg/m3 results in an average
concentration of 1.8 mg/m3 for the sampled MMU where the
applicable standard is 2.0 mg/m3. Despite the fact that two
of the measurements demonstrate noncompliance with a high degree of
confidence, corrective action would not have been required because the
average concentration was below the applicable standard.
As described in this notice and in conjunction with the MSHA/NIOSH
joint notice, under the new enforcement policy, whenever an individual
measurement indicates noncompliance (with a high level of confidence),
the mine operator will be required to take corrective action to lower
the concentration of respirable dust to comply with the applicable
standard.
Some commenters expressed concern that MSHA would fail to cite some
instances of noncompliance because of the high level of confidence
required for a citation. MSHA believes that the new enforcement
strategy as applied in conjunction with the finding of the MSHA/NIOSH
joint notice will reduce the chances of failing to cite cases of
noncompliance as compared to the previous policy of measurement
averaging, while at the same time ensuring that noncompliance is cited
only when there is a high degree of confidence that the applicable
standard has been exceeded. According to the inspector sampling
inspections conducted in 1995, only 132 MMUs were found to be in
violation of the applicable standard and cited under the previous
enforcement policy of measurement averaging, compared to 545 MMUs that
would have been citable under the new enforcement policy in conjunction
with the joint notice of finding using single, full-shift measurements.
This clearly demonstrates that the new enforcement policy, in
conjunction with the joint notice, will not compromise miners' health
but would, instead, have identified 413 additional instances of
overexposure that would have gone unaddressed under the previous policy
of measurement averaging.
Some commenters proposed that miners would be even more protected
if noncompliance was cited whenever any single, full-shift measurement
exceeded the applicable standard by any amount. That is, it was
recommended that MSHA not make any allowance for potential measurement
errors. MSHA has considered this recommendation but has not adopted it
in the final policy because it could result in citations being issued
where compliance with the applicable standard is more likely than not.
If the mine environment is sufficiently well controlled, it is more
likely that a particular measurement exceeds the applicable standard,
but not the CTV, due to measurement error rather than due to excessive
dust concentration. Furthermore, the rationale used by these commenters
to justify their proposed citation criterion breaks down when, as in
the case of multiple samples taken during a given shift in the same
MMU, more than one measurement is made for a single noncompliance
determination. Appendix D addresses technical details relating to this
issue.
Some commenters stated that MSHA's new citation criteria
implemented in conjunction with the joint notice will not improve
respirable dust levels in the environment, but will simply result in
MSHA issuing more citations to mine operators. In these commenters
view, this will foster a continuation of the adversarial relationship
that developed between mine operators and MSHA over allegations of
widespread tampering with respirable dust samples.
MSHA firmly believes that basing noncompliance determinations on a
single, full-shift measurement will improve working conditions for
miners because it will cause mine operators to either implement and
maintain more effective dust controls to minimize the chance of being
found in noncompliance by an MSHA inspector, or take corrective action
sooner to lower dust concentrations that are shown, with high
confidence, to be in excess of the applicable standard. The effect of
this new enforcement policy in conjunction with the MSHA/NIOSH joint
notice will be remedial in nature because it will address instances of
overexposure that are not addressed under the current policy of
measurement averaging. For example, between January 1992 and December
1993, MSHA continued the practice established under the SIP of making
noncompliance determinations based on single, full-shift measurements
which demonstrated, with high confidence, that the applicable standard
was exceeded, and on the average of multiple measurements. During this
period, MSHA inspectors issued a total of 658 citations at MMUs. The
majority of these citations (488) were issued based on the result of a
single, full-shift measurement. Under the existing enforcement policy,
such individual instances of noncompliance would not be cited and
corrected, but instead would be factored into an average that could be
at or below the applicable standard, resulting in no violation and no
corrective action taken by the mine operator.
Some commenters also contended that the joint notice of finding,
and this notice of policy, are solely for the administrative
convenience of MSHA's mine inspectors. The commenters stated that
allowing inspectors to make noncompliance determinations on the basis
of a single, full-shift measurement will eliminate the need for
inspectors to sample on successive days, as is sometimes required under
existing policy.
MSHA recognizes that there are administrative advantages related to
the adoption of this new enforcement policy and the joint notice of
finding. By eliminating the need to sample on subsequent days, the
Agency will be able to utilize its resources more efficiently. That is,
inspectors will not
[[Page 5693]]
be required to return to a mine to conduct additional dust sampling,
but the Agency will be able to redirect its resources to other safety
and health concerns. This result is consistent with the Mine Act's
objective of protecting miner safety and health. While administrative
convenience may be a side benefit of this new enforcement policy in
conjunction with the MSHA/NIOSH joint notice, the primary reason for
implementing it is to achieve the intent of Congress that no miner
shall be exposed to dust concentrations above the applicable standard
on any shift.
B. What is the Impact of the New Policy on Ventilation Plans?
A number of commenters expressed concern that issuing citations on
the result of a single, full-shift measurement will cause MSHA to
require carefully developed ventilation plans to be modified needlessly
as part of the abatement process. These commenters view such frequent
revisions as costly, disruptive and unnecessary. They contend that such
revisions, if required, would be made on the basis of incomplete or
invalid information, and that they would not necessarily decrease a
miner's dust exposure. Some commenters believed that some inspectors
would mandate specific changes without realistically evaluating their
effectiveness, while other inspectors would not allow operators to make
their own adjustments to the plans, or provide an opportunity for them
to evaluate the changes in a rational manner.
When a citation is issued based on a single measurement, this can
indicate that the control measures in use may no longer be adequate to
maintain the environment within the applicable standard. MSHA will
consequently review the adequacy of the ventilation plan under the
current operating conditions, and will consider the results of operator
bimonthly sampling as well as operator compliance with the approved
ventilation plan parameters. Under this approach MSHA would require
plan revisions only after an examination of all factors has
demonstrated that changes are necessary to protect miner health. This
enforcement strategy should minimize unnecessary changes to plans that
have been determined to provide adequate controls.
MSHA believes that the primary focus of the federal dust program is
to minimize miners' overexposures to respirable dust through the
application of appropriate environmental controls, which are stipulated
in the operator's approved mine ventilation plan. After these controls
are evaluated and shown to be effective under typical mining
conditions, if properly maintained, they should provide reasonable
assurance that no miner will be overexposed. Therefore, one of the
objectives of MSHA's dust sampling is to verify that the controls
stipulated in ventilation plans continue to adequately control dust
concentrations under existing operating conditions. In conjunction with
these sampling and other inspections an inspector checks and measures
the dust control parameters early in the shift to determine whether the
approved ventilation plan is being followed. A mine operator's failure
to follow the parameters stipulated in the plan will result in the
issuance of a citation, which requires immediate corrective action to
abate the violation. The type of corrective actions taken to abate plan
violations can vary from unplugging clogged water sprays to increasing
the amount of ventilating air delivered to the MMU. However, mere
correction of these deficiencies to ensure that the ``status quo'' of
the plan is being maintained may not always be effective in controlling
miners' exposure to respirable dust. The required plan parameters may
no longer be effective in maintaining compliance, and may need to be
upgraded. The determination of how the plan should be revised is
complicated by the fact that, generally, most approved plans do not
incorporate all the control measures that were in place when MSHA
sampled. Consequently, most plan revisions have simply incorporated
into the plan only those dust controls that were in use when MSHA
sampled, rather than requiring significant upgrading of the plan. As an
example, an MSHA inspector might require an increase in the water
pressure stipulated in the plan from 75 pounds per square inch (psi) to
125 psi to reflect the 125 psi that the MSHA inspector actually
measured. If, instead, the operator was required to significantly
increase the quantity of air being delivered to the MMU, this would be
considered a major upgrade. MSHA recognizes that a determination of
noncompliance should not automatically necessitate the revision of a
plan. Instead, it should result in a thorough review of the plan's
continued adequacy.
When an operator of an underground mine is cited for excessive
dust, 30 CFR 70.201(d) requires the operator to ``take corrective
action to lower the concentration of respirable dust to within the
permissible concentration.'' When the citation is based on MSHA
samples, the inspector may request that the operator describe what type
of corrective action will be taken. The inspector then determines if
the corrective action is appropriate. If it is not appropriate in the
specific situation, the inspector may either suggest or require other
corrective action or control measures. Operators are provided with the
opportunity to make adjustments to their dust controls and to evaluate
their effectiveness in a rational manner during the time for abatement
set by the inspector, which is based on the complexity of the problem,
availability of controls, and the types of changes the operator intends
to make. This abatement time may be extended by the inspector based on
the operator's performance in reducing the dust concentration in the
affected area of the mine. Typically, the operator then demonstrates,
through sampling, that the underlying condition or conditions causing
the violation have been corrected. Failure to take corrective action
prior to sampling that shows continuing noncompliance may lead to the
issuance of a withdrawal order. However, this occurs infrequently.
C. Will the New Enforcement Policy Increase Citations on Individual
Shifts, Even if the So-Called ``Average Concentration Over the Longer
Term'' Meets the Standard?
Some commenters claimed that even when the average dust
concentration is well below the applicable standard, normal variability
from shift to shift results in a substantial fraction of shifts for
which the dust standard is exceeded. According to these commenters, a
determination of noncompliance is warranted only if the average dust
concentration to which a miner is exposed exceeds the standard over a
period of time greater than a single shift, such as a bimonthly
sampling period, a year, or a miner's working lifetime. Therefore, they
consider it ``unfair'' to cite operators for exceeding the applicable
standard on individual shifts, so long as the average over the longer
term meets the applicable standard. For example, based on historical
sampling data provided by one commenter, the commenter concluded that,
``* * * there is at least a 1 in 6 or 17% probability that any single
sample can show potential overexposure when one does not exist.'' These
commenters contend that use of the CTV to determine noncompliance,
based on one sample collected on a single shift, will substantially
increase the frequency of ``unfair'' citations, compared to existing
MSHA policy.
MSHA believes that such comments reflect a misunderstanding of both
the
[[Page 5694]]
requirements of the Mine Act and MSHA's longstanding policy with
respect to single, full-shift noncompliance determinations. It should
be recognized that MSHA has been basing noncompliance determinations on
the average of multiple occupation measurements obtained on the same
shift since 1975. In addition, some of the commenters confused the
average dust concentration over the course of an individual shift with
the average dust concentration over some longer term. The joint notice
of finding issued by the Secretaries of Labor and HHS addresses this
issue. Since the Mine Act requires that dust concentration be kept
continuously at or below the applicable standard on every shift, it is
appropriate to cite noncompliance when any single, full-shift
measurement at a particular location demonstrates, with high
confidence, that the applicable standard has been exceeded on an
individual shift.
Section 201(b) of the Mine Act mandates that MSHA ensure ``to the
greatest extent possible, that the working conditions in each
underground coal mine are sufficiently free of respirable dust
concentrations * * * to permit each miner the opportunity to work
underground during the entire period of his adult life without
incurring any disability from pneumoconiosis or any other occupation-
related disease during or at the end of such a period.'' Since neither
past nor future exposure levels can be assumed for any miner, MSHA's
enforcement strategy must be to limit the exposure on every shift as
intended by the Mine Act.
D. Will There Be Any Changes in Operator Bimonthly Sampling?
Several commenters were unclear about the impact of the joint MSHA/
NIOSH finding and this policy on operator sampling for compliance and
for abatement of violations. One commenter suggested that 30 CFR
70.207(a) be revised to allow the operator to submit one single, full-
shift sample, instead of five samples every bimonthly period as
currently required. Another commenter suggested that MSHA assume
responsibility for dust sampling from the mine operators.
MSHA has previously noted that the change in its enforcement policy
announced through this final notice affects only how it will determine
noncompliance based on measurements obtained by MSHA inspectors. There
will be no change in how MSHA evaluates operator-collected respirable
dust samples for compliance. Under the regulations currently in effect,
the Agency will continue to average operator samples taken on multiple
shifts or days to make noncompliance determinations. MSHA is committed
to revising procedures with respect to operator-collected respirable
dust samples through the rulemaking process for consistency with this
final finding.
Several commenters expressed concerns about the credibility of the
operator sampling program because of alleged operator tampering with
respirable dust samples and alleged operator manipulation of mine
conditions during dust sampling periods. As a result, these commenters
felt that mine operators should no longer have responsibility for
sampling because their sampling results are unreliable. Another
commenter expressed support for the Agency to compel coal mine
operators to comply with existing dust standards. Another commenter
voiced concern that a mine operator could be wrongly cited due to the
loss or mishandling of a single, full-shift sample by MSHA, and claimed
that such occurrences had happened in the past. Some commenters believe
that if noncompliance can be determined based on a single, full-shift
sample, an operator should be allowed to abate a citation with a
single, full-shift sample, particularly if the operator has recently
demonstrated compliance through bimonthly samples. Another commenter
questioned the impact of the proposed program on the operator's
program, specifically, whether MSHA would require each of the abatement
samples to meet the single, full-shift sample citation threshold
values, in addition to meeting the dust standard based on the average
of five abatement samples.
Issues concerning operator sampling are not germane to this
enforcement policy notice, which concerns only the use of samples
collected by MSHA inspectors. The changes set forth in this final
notice only address how MSHA will determine noncompliance when sampling
is conducted by federal mine inspectors. There is no change in how MSHA
evaluates either operator-collected bimonthly samples or samples taken
to abate a dust citation. MSHA is committed to revising any procedures
with respect to the operator program through the rulemaking process for
consistency with this final finding.
Concerning the credibility of the operator sampling program, MSHA
recognizes that there have been instances of abuse under the current
operator sampling program. The Task Group found that the majority of
operators do not engage in such conduct. MSHA will continue to monitor
the operator sampling program, increase the frequency of inspector
sampling, and target problem mines for additional inspections, as
appropriate.
MSHA processes over 80,000 samples annually and it is not
unrealistic to expect some samples to be either lost in the mail or
accidentally misplaced. MSHA's experience of processing more than 7
million dust samples since 1970 indicates that this occurs
infrequently. In the event a sample is lost, the mine operator is
afforded ample opportunity to submit a replacement sample. If a
citation is issued due to the operator's failure to submit the required
number of samples, the affected operator can present evidence that the
required number of samples had been submitted and request that MSHA
vacate the citation.
E. How Can MSHA Base a Noncompliance Determination on a Single, Full-
Shift Sample, When Five Samples Are Required in Operator Bimonthly
Sampling?
Once a finding has been made that a single, full-shift measurement
will accurately represent atmospheric conditions to which a miner is
exposed during such shift, MSHA is bound by the terms of the Mine Act
to make noncompliance determinations based on single, full-shift
measurements. No regulatory action is required to implement this change
in MSHA's dust sampling program. On the other hand, the present
regulatory scheme for operator sampling was developed based on
noncompliance determinations being made by averaging the results of
multiple samples over five successive shifts or days. In order for MSHA
to incorporate the single, full-shift sample concept into the operator
sampling program, the Agency must revise the operator sampling
regulations through notice and comment rulemaking.
F. Do the New Citation Criteria Have any Impact on Permissible Exposure
Limits?
Some commenters contended that a policy of citing in accordance
with the CTV table, rather than citing whenever a measurement exceeds
the applicable standard, effectively increases the allowable dust
concentration limit. Other commenters stated that the enforcement of
the applicable standard as a limit on each shift, rather than as a
limit on the average concentration over some longer time period,
effectively reduces the standard.
Citing in accordance with the stated CTV neither increases nor
decreases the dust standard. Operators are required to maintain
compliance with the
[[Page 5695]]
applicable standard at all times. MSHA's citing of noncompliance only
when there is high confidence that the applicable standard has been
exceeded does not increase the permissible concentration limit. Again,
mine operators must maintain compliance with the applicable standard.
MSHA requires that dust controls maintain dust concentrations at or
below the applicable standard on all shifts, not merely at or below the
CTV. It is also MSHA's intent under this new enforcement policy that if
a measurement exceeds the applicable standard by an amount insufficient
to warrant citation--that is, the level does not meet or exceed the
CTV--MSHA will target that mine or area for additional sampling to
ensure that dust controls are adequate.
Those commenters who stated that applying the applicable standard
to each shift will effectively reduce the respirable dust standard
overlooked the fact that, since 1975, MSHA has taken enforcement action
based on average of measurements obtained for different occupations
during a single shift. This new enforcement policy does not change
MSHA's interpretation of section 202(b) of the Mine Act that dust
concentrations be maintained at or below the applicable standard on
each shift. The new enforcement policy merely reflects a change in the
technical criteria used to cite violations of the applicable dust
standard.
Appendix A--The Effects of Averaging Dust Concentration Measurements
MSHA's measurement objective in collecting a dust sample is to
determine the average dust concentration at the sampling location on
the shift sampled. As discussed in the joint notice of finding
published elsewhere in today's Federal Register, a single, full-shift
measurement can accurately represent the average full-shift dust
concentration being measured. Nevertheless, because of sampling and
analytical errors inherent in even the most accurate measurement
process, the true value of the average dust concentration on the
sampled shift can never be known with complete certainty. However
accurate the representation, a measurement can provide only an estimate
of the true dust concentration. Some commenters contended that MSHA
should not rely on single samples for making noncompliance
determinations, because an average of results from multiple samples
would estimate the true dust concentration more accurately than any
single measurement.
Contrary to the views expressed by these commenters, averaging a
number of measurements does not necessarily improve the accuracy of an
estimation procedure. Consider, for example, an archer aiming at
targets mounted at random and possibly overlapping positions on a long
partition. Each arrow might be aimed at a different target. Suppose
that an observer, on the opposite side of the partition from the
archer, cannot see the targets but must estimate the position of each
bull's eye by locating protruding arrowheads.
Each protruding arrowhead provides a measurement of where some
bull's eye is located. If two arrowheads are found on opposite ends of
the partition, averaging the positions of these two arrowheads would
not be a good way of determining where any real target is located. To
estimate the location of an actual target, it would generally be
preferable to use the position of a single arrow. The average would
represent nothing more than a ``phantom'' target somewhere near the
center, where the archer probably did not aim on either shot and where
no target may even exist.
The archery example can be extended to illustrate conditions under
which averaging dust concentration measurements does or does not
improve accuracy. If each arrowhead is taken to represent a full-shift
dust sample, then the true average dust concentration at the sampling
location on a given shift can be identified with the location of the
bull's eye at which the corresponding arrow was aimed. The accuracy of
a measurement refers to how closely the measurement can be expected to
come to the quantity being measured. Statistically, accuracy is the
combination of two distinct concepts: precision, which pertains to the
consistency or variability of replicated measurements of exactly the
same quantity; and bias, which pertains to the average amount by which
these replicated measurements deviate from the quantity being measured.
Bias and precision are equally important components of measurement
accuracy.
To illustrate, arrows aimed at the same target might consistently
hit a sector on the lower right side of the bull's eye. The protruding
arrowheads would provide more or less precise measurements of where the
bull's eye was located, depending on how tightly they were clustered;
but they would all be biased to the lower right. On the other hand, the
arrows might be distributed randomly around the center of the bull's
eye, and hence unbiased, but spread far out all over the target. The
protruding arrowheads would then provide unbiased but relatively
imprecise measurements.
More complicated situations can easily be envisioned. Arrows aimed
at a second target would provide biased measurements relative to the
first target. Alternatively, if the archer always aims at the same
target, the first shot in a given session might tend to hit near the
center, with successive shots tending to fall off further and further
to the lower right as the archer's arm tires; or shots might
progressively improve, as the archer adjusts aim in response to prior
results.
Averaging reduces the effects of random errors in the archer's aim,
thereby increasing precision in the estimation procedure. If the archer
always aims at the same target and is equally adept on every shot
(i.e., if the arrowheads are all randomly and identically distributed
around a fixed point), then averaging improves the estimate's precision
without introducing any bias. Averaging in such cases provides a more
accurate method of estimating the bull's eye location than reliance on
any single arrowhead. If, however, the archer intentionally or
unintentionally switches targets, or if the archer's aim progressively
deteriorates, then averaging can introduce or increase bias in the
estimate. If the gain in precision outweighs this increase in bias,
then averaging several independent measurements may still improve
accuracy. However, averaging can also introduce a bias large enough to
offset or even surpass the improvement in precision. In such cases, the
average position of several arrowheads can be expected to locate the
bull's eye less accurately than the position of a single arrowhead.
I. Multi-Locational Averaging
Some commenters opposed MSHA's use of a single, full-shift
measurement for enforcement purposes, claiming that determinations
based on such measurements would be less accurate than those made under
MSHA's existing enforcement policy of averaging multiple measurements
taken on an MMU. There are two distinctly different types of multi-
locational measurement averages that could theoretically be compiled on
a given shift: (1) the average might combine measurements taken for
different occupational locations and (2) the average might combine
measurements all taken for the same occupational location. For MMUs,
the averages used in MSHA's sampling program usually involve
measurements taken for different occupational locations on the same
shift. These are averages of the first type. MSHA's sampling program
has never utilized
[[Page 5696]]
averages of the second type. Therefore, those commenters who claimed
that reliance on a single, full-shift measurement would reduce the
accuracy of noncompliance determinations, as compared to MSHA's
existing enforcement policy, are implicitly claiming that accuracy is
increased by averaging across different occupational locations.
Averaging measurements obtained from different occupational
locations on an MMU is like averaging together the positions of arrows
aimed at different targets. The average of such measurements is an
artificial, mathematical construct that does not correspond to the dust
concentration for any actual occupational location. Therefore, this
type of averaging introduces a bias proportional to the degree of
variability in actual dust concentration at the various locations
averaged.
The gain in precision that results from averaging measurements
taken at different locations outweighs this bias only if variability
from location to location is smaller than variability in measurement
error. However, commenters opposed to MSHA's use of single, full-shift
measurements for enforcement purposes argued that this is not generally
the case and even submitted data and statistical analyses in support of
this position. Commenters in favor of noncompliance determinations
based on a single, full-shift measurement agreed that variability in
dust concentration is extensive for different occupational locations
and argued that MSHA's existing policy of measurement averaging is not
sufficiently protective of miners working at the dustiest locations.
Since an average of the first type combines measurement from the
dustiest location with measurements from less dusty locations, it must
always fall below the best available estimate of dust concentration at
the dustiest location. In effect, averaging across different
occupational locations dilutes the dust concentration observed for the
most highly exposed occupations or dustiest work positions. Therefore,
such averaging results in a systematic bias against detecting excessive
dust concentrations for those miners at greatest risk of overexposure.
A somewhat better case can be made for the second type of multi-
locational averaging, which combines measurements obtained on the same
shift from a single occupational location. As some commenters pointed
out, however, there is ample evidence that spatial variability in dust
concentration, even within relatively small areas, is frequently much
larger than variability due to measurement error. Therefore, the same
kind of bias introduced by averaging across occupational locations
would also arise, but on a lesser scale, if the average measurement
within a relatively small radius were used to represent dust
concentration at every point in the atmosphere to which a miner is
exposed. A miner is potentially exposed to the atmospheric conditions
at any valid sampling location. Consistent with the Mine Act and
implementing regulations, MSHA's enforcement strategy is to limit
atmospheric dust concentration wherever miners normally work or travel.
Therefore, the more spatial variability in dust concentration there is
within the work environment, the less appropriate it is to use
measurement averaging to enforce the applicable standard by averaging
measurements obtained at different sampling locations.
Some of the comments implied that instead of measuring average dust
concentration at a specific sampling location, MSHA's objective should
be to estimate the average dust concentration throughout a miner's
``breathing zone'' or other area near a miner. If estimating average
dust concentration throughout some zone were really the objective of
MSHA's enforcement strategy, then averaging measurements made at random
points within the zone would improve precision of the estimate without
introducing a bias. This type of averaging, however, has never been
employed in either the MSHA or operator dust sampling programs. MSHA's
current policy of averaging measurements obtained from different zones
does not address spatial variability in the area immediately
surrounding a sampler unit. Therefore, even if averaging measurements
from within a zone were somehow beneficial, this would not demonstrate
that MSHA's existing enforcement policy is more reliable than the new
policy of basing noncompliance on a single, full-shift measurement.
Furthermore, if MSHA's objective were really to estimate average
dust concentration throughout some specified zone on a given shift,
then it would be necessary to obtain far more than five simultaneous
measurements within the zone. This is not only because of potentially
large local differences in dust concentration. In order to use such
measurements for enforcement purposes, variability in dust
concentration within the sampled area would have to be estimated along
with the average dust concentration itself. As some commenters
correctly pointed out, doing this in a statistically valid way would
generally require at least twenty to thirty measurements. One of these
commenters also pointed out that such an estimate, based on even this
many measurements in the same zone, could be regarded as accurate only
under certain questionable assumptions about the distribution of dust
concentrations. This commenter calculated that hundreds of measurements
would be required in order to avoid these tenuous assumptions. Clearly,
this shows that the objective of estimating average dust concentration
throughout a zone is not consistent with any viable enforcement
strategy to limit dust concentration on each shift in the highly
heterogeneous and dynamic mining environment. The large number of
measurements required to accurately characterize dust concentration
over even a small area merely demonstrates why it is not feasible to
base enforcement decisions on estimated atmospheric conditions beyond
the sampling location.
MSHA recognizes that a single, full-shift measurement will not
provide an accurate estimate of average dust concentration anywhere
beyond the sampling location. The Mine Act, however, does not require
MSHA to estimate average dust concentration at locations that are not
sampled or to estimate dust concentration averaged over any zone or
region of the mine, and doing so is not part of MSHA's enforcement
program. Instead, MSHA's enforcement strategy is to ensure that a miner
will not be exposed to excessive dust wherever he/she normally works or
travels. This is accomplished by maintaining the average dust
concentration at each valid sampling location at or below the
applicable standard during each shift.
II. Multi-Shift Averaging
Some commenters maintained that in order to reduce the risk of
erroneous noncompliance determinations, MSHA should average
measurements obtained from the same occupation on different shifts.
These commenters contended that the average of measurements from
several shifts represents the average dust concentration to which a
miner is exposed more accurately than a single, full-shift measurement.
Other commenters, who favored noncompliance determinations based on
single, full-shift measurements, claimed that conditions are sometimes
manipulated so as to produce unusually low dust concentrations on some
of the sampled shifts. These commenters suggested that, due to these
[[Page 5697]]
unrepresentative shifts, multi-shift averaging can yield
unrealistically low estimates of the dust concentration to which a
miner is typically exposed. Some of these commenters also argued that
the Mine Act requires the dust concentration to be regulated on each
shift, and that multi-shift averaging is inherently misleading in
detecting excessive dust concentration on an individual shift.
Those advocating multi-shift averaging generally assumed that a
noncompliance determination involves estimating a miner's average dust
exposure over a period longer than an individual shift. This assumption
is flawed because section 202(b) of the Mine Act specifies that each
operator shall continuously maintain the average concentration of
respirable dust in the mine atmosphere during each shift at or below
the applicable standard. Some of those advocating multi-shift
averaging, however, suggested that MSHA should average measurements
obtained on different shifts even if the quantity of interest is dust
concentration on an individual shift. These commenters argued that
averaging smooths out the effects of measurement errors, and that
therefore the average over several shifts would represent dust
concentration on each shift more accurately than the corresponding
individual, full-shift measurement.
The Secretary recognizes that there are circumstances, not
experienced in mining environments, under which averaging across shifts
could improve the accuracy of an estimate for an individual shift. Just
as averaging the positions of arrows aimed at nearly coinciding targets
might better locate the bull's eye than the position of any individual
arrow, the gain in precision obtained by averaging dust concentrations
observed on different shifts could, under analogous circumstances,
outweigh the bias introduced by using the average to estimate dust
concentration for an individual shift. This would be the case, however,
only if variability in dust concentration among shifts were small
compared to variability due to measurement imprecision. It would do no
good to average the location of arrows aimed at different targets
unless the targets were at nearly identical locations.
To the contrary, several commenters pointed out that variability in
dust concentration from shift to shift tends to be much larger than
variability due to measurement error and introduced evidence in support
of this observation. Measurements on different shifts are like arrows
aimed at widely divergent targets. The more that conditions vary, for
any reason, from shift to shift, the more bias is introduced by using a
multi-shift average to represent dust concentration for any individual
shift. Under these circumstances, any improvement in precision to be
gained by simply averaging results is small compared to the bias
introduced by such averaging. Therefore, the Secretary has concluded
that MSHA's existing practice of averaging measurements collected on
different shifts does not improve accuracy in estimating dust
concentration to which a miner is exposed on any individual shift. To
paraphrase one commenter, averaging Monday's exposure measurement with
Tuesday's does not improve the estimate of Monday's average dust
concentration.
Some commenters argued that since the risk of pneumoconiosis
depends on cumulative exposure, MSHA's objective should be to estimate
the dust concentration to which a miner is typically exposed and to
identify cases of excessive dust concentration over a longer term than
a single shift. Other commenters claimed that a multi-shift average
does not provide a good estimate of either typical dust concentrations
or exposures over the longer term. These commenters claimed that
different shifts are not equally representative of the usual
atmospheric conditions to which miners are exposed, implying that the
average of measurements made on different shifts of a multi-day MSHA
inspection tends to systematically underestimate typical dust
concentrations.
The Secretary interprets section 202(b) of the Mine Act as
requiring that dust concentrations be kept at or below the applicable
standard on each and every shift. Nevertheless, the Secretary
recognizes that, under certain conditions, the average of measurements
from multiple shifts can be a better estimate of ``typical''
atmospheric conditions than a single measurement. This applies,
however, only if the sampled shifts comprise a random or representative
selection of shifts from whatever longer term may be under
consideration. As shown below, evidence to the contrary exists,
supporting those commenters who maintained that measurements collected
over several days of a multi-day MSHA inspections do not meet this
requirement. Therefore, the Secretary has concluded that averaging such
measurements is likely to be misleading even for the purpose of
estimating dust concentrations to which miners are typically exposed.
Whether the objective is to measure average dust concentration on
an individual shift or to estimate dust concentration typical of a
longer term, the arguments presented for averaging across shifts all
depend on the assumption that every shift sampled during an MSHA
inspection provides an unbiased representation of dust exposure over
the time period of interest.1 To check this assumption, MSHA
performed a statistical analysis of multi-shift MSHA inspections
carried out prior to the SIP. This analysis, placed into the record in
September 1994, examined the pattern of dust concentrations measured
over the course of these multi-shift inspections and compared results
from the final shift with results from a subsequent single-shift
sampling inspection [1].
---------------------------------------------------------------------------
\1\ Technically, the assumption is that dust concentrations on
all shifts sampled are independently and identically distributed
around the quantity being estimated.
---------------------------------------------------------------------------
The analysis found that dust concentrations measured on different
shifts of the same MSHA inspection were not randomly distributed. The
later samples tended to show significantly lower results than earlier
samples, indicating that dust concentrations on later shifts of a
single inspection may decline in response to the presence of an
inspector. Furthermore, the analysis provided evidence that the
reduction in dust concentration tends to be reversed after the
inspection is terminated. These two results led to the conclusion that
averaging dust concentrations measured on different shifts of a multi-
day MSHA inspection introduces a bias toward unrealistically low dust
concentrations.
One commenter questioned the validity of this analysis, stating
that ``there is absolutely no basis in the * * * report for the
assertion that the trend is reversed after the inspection is
terminated.'' This commenter apparently overlooked Table 3 of the
report. That table shows a statistically significant reversal at those
mine entities included in the analysis that were subsequently inspected
under MSHA's SIP. Dust concentrations measured at these mine entities
had declined significantly between the first and last days of the
multi-shift inspection. It was primarily to address the commenter's
implication that these reductions reflected permanent ``adjustments in
dust control measures'' that the analysis included a comparison with
the subsequent SIP inspection. An increase, representing a reversal of
the previous trend, was observed on the single shift of the subsequent
[[Page 5698]]
inspection, relative to the dust concentration measured on the final
shift of the previous multi-shift inspec tion. This reversal was found
to be ``statistically significant at a confidence level of more than
99.99 percent.''
The same commenter also stated that MSHA ``* * * fails to address
the systematic [selection] bias of the study. MSHA only does multiple
day sampling when the initial results are higher, but not out of
compliance.'' It is true that in order to be selected for revisitation,
a mine entity must have shown relatively high concentrations on the
first shift--though not, in the case of an MMU, so high as to warrant a
citation on first shift. Since no experimental data were available on
mine entities randomly selected to receive multi-shift inspections, the
only cases in which patterns over the course of a multi-shift
inspection could be examined were cases selected for multi-shift
inspection under these criteria.
Although the impact of the selection criteria was not explicitly
addressed, it was recognized that entities selected for multi-day
inspections do not constitute a random selection of mine entities. This
recognition motivated, in part, the report's comparison of the final
shift measurement to the dust concentration measured during a
subsequent single-shift inspection. The magnitude of the average
reversal indicates that most of the reduction observed over the course
of the multi-shift inspection cannot be attributed to the selection
criteria. Furthermore, it was not only mine entities with relatively
low dust concentration measurements that were left out of the study
group. Mine entities with the highest dust concentration measurements
were immediately cited based on the average of measurements taken and
excluded from the group subjected to multi-shift dust inspections.
Therefore, the effect on the analysis of selecting mine entities with
relatively high initial dust concentration measurements was largely
offset by the effect of excluding those entities with even higher
initial measurements. In any event, the magnitude of the average
reduction between first and last shifts of a multi-shift inspection was
significantly greater than what can be explained by selection for
revisitation due to measurement error on the first shift sampled.
The assumption that multiple shifts sampled during a single MSHA
inspection are equally representative is clearly violated if, as some
commenters alleged, operating conditions are deliberately altered after
the first shift in response to the continued presence of an MSHA
inspector and then changed back after the inspector leaves. However, if
samples are collected on successive or otherwise systematically
determined shifts or days, the assumption can also be violated by
changes arising as part of the normal mining cycle. As one commenter
pointed out, multi-shift averaging within a single MSHA inspection
potentially introduces biases typical of ``campaign sampling,'' in
which observations of a dynamic process are clustered together over a
relatively narrow time span. In order to construct an unbiased, multi-
shift average for each phase of mining activity, it would be necessary
to collect samples from several shifts operating under essentially the
same conditions. Alternatively, to construct an unbiased, multi-shift
estimate of dust concentration over a longer term, it would be
necessary to collect samples from randomly selected shifts over a
period great enough to reflect the full range of changing conditions.
Neither requirement is met by multi-shift MSHA inspections because (1)
the mine environment is dynamic and no two shifts are alike and (2)
MSHA inspectors are not there long enough to observe every condition in
their inspection.
Based on the analysis presented by Kogut [1] and also on public
comments received in response to the February 18 and June 6, 1994,
notices, the Secretary has concluded that it should not be assumed that
multiple shifts sampled during a single MSHA inspection are equally
representative of atmospheric conditions to which a miner is typically
exposed. This conclusion undercuts the rationale for multi-shift
averaging within a single MSHA inspection, regardless of whether the
objective is to estimate dust concentration for the individual shifts
sampled as it is for MSHA inspector sampling or for typical shifts over
a longer term as implied by some commenters. Measurements collected by
MSHA on consecutive days or shifts of the same inspection do not
comprise a random or otherwise representative sample from any larger
population of shifts that would properly represent a long-term exposure
or a particular phase of the mining cycle. Therefore, there is no basis
for assuming that multi-shift averaging improves accuracy or reduces
the risk of an erroneous enforcement determination.
Appendix B--Citation Threshold Values (CTV)
I. Interpretation of the CTV Table
Each CTV was calculated to ensure that, if the CTV is met or
exceeded, noncompliance with the applicable standard can be inferred
with at least 95-percent confidence. It is assumed that whatever dust
standard happens to be in effect at the sampling location is binding,
and that a citation is warranted whenever there is sufficient evidence
that an established standard has been exceeded. The CTV table does not
depend on how the applicable standard was established, or on any
measurement uncertainties in the process of setting the applicable
standard.
Some commenters argued that in order to construct a valid table of
CTVs, MSHA would have to take into account the statistical distribution
of dust concentrations over many shifts and locations. One commenter
suggested that stochastic properties of the dust concentrations, which
describe variability over time in probabilistic terms, should also be
taken into account. MSHA, however, intends to use single, full-shift
measurements only in determining noncompliance with the applicable
standard on a particular shift and at the sampling location consistent
with the measurement objective described in the MSHA and NIOSH joint
finding published elsewhere in today's Federal Register. This is
analogous to using a single measurement to identify individual
suitcases that are unacceptable because they weigh more than five
pounds. The efficacy of using a single measurement to identify
unacceptable suitcases depends on the accuracy of the scale and the
skill of the weigher. It does not depend on the statistical
distribution of weights among suitcases or on any stochastic properties
of the suitcase production process. These considerations would be
relevant to estimating average weight for all suitcases produced, but
they have nothing whatsoever to do with determining the weight of an
individual suitcase using a sufficiently accurate scale. Averaging the
weights of several suitcases would be entirely inappropriate and
extremely misleading, since the object is to identify individual
suitcases weighing more than five pounds. Although the measured weight
of an individual suitcase is liable to contain some error (so the
decision might be uncertain for a suitcase weighing five pounds and one
ounce), a suitcase weighing seven or eight pounds could be rejected
with high confidence on the first weighing. Additional weighings (of
the same suitcase) would be required only for those suitcases whose
initial measurement was very close to five pounds.
The CTV table provides criteria for testing a tentative, or
presumptive,
[[Page 5699]]
hypothesis that the true full-shift average dust concentration did not
exceed the applicable standard (S) at each of the individual locations
sampled during a particular shift. For purposes of this test, the mine
atmosphere at each such location is presumed to be in compliance unless
the corresponding full-shift measurement provides sufficient evidence
to the contrary. The ``true full-shift average'' does not refer, in
this context, to an average across different occupations, locations, or
shifts. Instead, it refers entirely to the dust concentration at the
specific location of the sampler unit, averaged over the course of the
particular shift during which the measurement was obtained. The CTV
table is not designed to estimate or test the average dust
concentration across occupational locations, or within any zone or mine
area, or in the air actually inhaled by any particular miner.
Some commenters questioned why more than one sample might be
required, if the first sample collected does not exceed the CTV. One of
these commenters argued that in such case, ``compliance has already
been established at a 95% confidence level based on the first single
shift sample.'' This line of argument confuses confidence in issuing a
citation with confidence of compliance. It also shows a basic
misunderstanding of how the citation criteria relate to the requirement
of continuous compliance under section 202(b) of the Mine Act.
The CTV table ensures that noncompliance is cited only when there
is a 95-percent level of confidence that the applicable standard has
actually been exceeded. If a single measurement does not meet the
criterion for citation, this does not necessarily imply probable
compliance with the dust standard--let alone compliance at a 95-percent
confidence level. For example, a single, full-shift measurement of 2.14
mg/m3 would not, according to the CTV table, indicate
noncompliance with sufficient confidence to warrant a citation if S =
2.0 mg/m3. This does not imply that the mine atmosphere was
in compliance on the shift and at the location sampled. On the
contrary, unless contradictory evidence were available, this
measurement would indicate that the MMU was probably out of compliance.
However, because there is a small chance that the measurement exceeded
the standard only because of measurement error, a citation would not be
issued. Additional measurements would be necessary to verify the
apparent lack of adequate control measures. Similarly, a single, full-
shift measurement of 1.92 mg/m3 would not warrant citation;
but, because of possible measurement error, neither would it warrant
concluding that the mine atmosphere sampled was in compliance. To
confirm that control measures are adequate, it would be necessary to
obtain additional measurements.
Furthermore, even if a single, full-shift measurement were to
demonstrate, at a high confidence level, that the mine atmosphere was
in compliance at the sampling location on a given shift, additional
measurements would be required to demonstrate compliance on each shift.
For example, if S = 2.0 mg/m3, then a valid measurement of
1.65 mg/m3 would demonstrate compliance on the particular
shift and at the particular location sampled. It would not, however,
demonstrate compliance on other shifts or at other locations.
II. Derivation of the CTV Table
Some commenters requested an explanation of the statistical theory
underlying the CTV table. To understand how the CTVs are derived and
justified, it is first necessary to distinguish between variability due
to measurement error and variability due to actual differences in dust
concentration. The variability observed among individual measurements
obtained at different locations (or at different times) combines both:
dust concentration measurements vary partly because of measurement
error and partly because of genuine differences in the dust
concentration being measured. This distinction, between measurement
error and variation in the true dust concentration, can more easily be
explained by first carefully defining some notational abbreviations.
One or more dust samples are collected in the same MMU or other
mine area on a particular shift. Since it is necessary to distinguish
between different samples in the same MMU, let Xi represent
the MRE-equivalent dust concentration measurement obtained from the
ith sample. The quantity being measured is the true, full-
shift average dust concentration at the ith sampling
location and is denoted by i. Because of potential
measurement errors, i can never be known with
complete certainty. A ``sample,'' ``measurement,'' or ``observation''
always refers to an instance of Xi rather than
i.
The overall measurement error associated with an individual
measurement is nothing more than the difference between the measurement
(Xi) and the quantity being measured
(i). Therefore, this error can be represented as
i = Xi-i.
Equivalently, any measurement can be regarded as the true concentration
in the atmosphere sampled, with a measurement error added on:
Xi = i + i.
For two different measurements (X1 and X2), it
follows that X1 may differ from X2 not only
because of the combined effects of 1 and
2, but also because 1 differs
from 2.
The probability distribution of Xi around
i depends only on the probability distribution of
i and should not be confused with the statistical
distribution of i itself, which arises from spatial
and/or temporal variability in dust concentration. This variability
[i.e., among i for different values of I] is not
associated with inadequacies of the measurement system, but real
variation in exposures due to the fact that contaminant generation
rates vary greatly in time and contaminants are heterogeneously
distributed in workplace air.
Since noncompliance determinations are made relative to individual
sampling locations on individual shifts, derivation of the CTV table
requires no assumptions or inferences about the spatial or temporal
pattern of atmospheric dust concentrations--i.e., the statistical
distribution of i. MSHA is not evaluating dust
concentrations averaged across the various sampler locations.
Therefore, the degree and pattern of variability observed among
different measurements obtained during an MSHA inspection are not used
in establishing any CTV. Instead, the CTV for each applicable standard
(S) is based entirely on the distribution of measurement errors
(i) expected for the maximum dust concentration in
compliance with that standard--i.e., a concentration equal to S itself.
If control filters are used to eliminate potential biases, then
each i arises from a combination of four weighing
errors (pre-and post-exposure for both the control and exposed filter
capsule) and a continuous summation of instantaneous measurement errors
accumulated over the course of an eight-hour sample. Since the eight-
hour period can be subdivided into an arbitrarily large number of sub-
intervals, and some fraction of i is associated
with each sub-interval, i can be represented as
comprising the sum of an arbitrarily large number of sub-interval
errors. By the Central Limit Theorem, such a summation tends to be
normally distributed, regardless of the distribution of subinterval
errors. This does not depend on the distribution of
[[Page 5700]]
i, which is generally represented as being
lognormal.
Furthermore, each measurement made by an MSHA inspector is based on
the difference between pre- and post-exposure weights of a dust sample,
as determined in the same laboratory, and adjusted by the weight gain
or loss of the control filter capsule. Any systematic error or bias in
the weighing process attributable to the laboratory is mathematically
canceled out by subtraction. Furthermore, any bias that may be
associated with day-to-day changes in laboratory conditions or
introduced during storage and handling of the filter capsules is also
mathematically canceled out. Elimination of the sources of systematic
errors identified above, together with the fact that the concentration
of respirable dust is defined by section 202(e) of the Mine Act to mean
the average concentration of respirable dust measured by an approved
sampler unit, implies that the measurements are unbiased. This means
that i is equally likely to be positive or negative
and, on average, equal to zero.
Therefore, each i is assumed to be normally
distributed, with a mean value of zero and a degree of variability
represented by its standard deviation
[GRAPHIC] [TIFF OMITTED] TN31DE97.012
Since Xi = i + i, it
follows that for a given value of i, Xi
is normally distributed with expected value equal to
i and standard deviation equal to
i. CVtotal, described in the MSHA and
NIOSH joint finding published elsewhere in today's Federal Register, is
the coefficient of variation in measurements corresponding to a given
value of i. CVtotal relates entirely to
variability due to measurement errors and not at all to variability in
actual dust concentrations.
MSHA's procedure for citing noncompliance based on the CTV table
consists of formally testing a presumption of compliance at every
location sampled. Compliance with the applicable standard at the
ith sampling location is expressed by the relation
i S. Max{i} denotes
the maximum dust concentration, among all of the sampling locations
within an MMU. Therefore, if Max{i} S,
none of the sampler units in the MMU were exposed to excessive dust
concentration. Since the burden of proof is on MSHA to demonstrate
noncompliance, the hypothesis being tested (called the null hypothesis,
or H0,) is that the concentration at every location sampled
is in compliance with the applicable standard. Equivalently, for an MMU
the null hypothesis (H0) is that max{i}
S. In other areas, where only one, full-shift measurement
is made, the null hypothesis is simply that i
S.
The test consists of evaluating the likelihood of measurements
obtained during an MSHA inspection, under the assumption that
H0 is true. Since Xi = i +
i, Xi (or max{Xi} in the case
of an MMU) can exceed S even under that assumption. However, based on
the normal distribution of measurement errors, it is possible to
calculate the probability that a measurement error would be large
enough to fully account for the measurement's exceeding the standard.
The greater the amount by which Xi exceeds S, the less
likely it is that this would be due to measurement error alone. If,
under H0, this probability is less than five percent, then
H0 can be rejected at a 95-percent confidence level and a
citation is warranted. For an MMU, rejecting H0 (and
therefore issuing a citation) is equivalent to determining that
i S for at least one value
of I.
Each CTV listed was calculated to ensure that citations will be
issued at a confidence level of at least 95 percent. As described in
MSHA's February 1994 notice and explained further by Kogut [2], the
tabled CTV corresponding to each S was calculated on the assumption
that, at each sampling location:
[GRAPHIC] [TIFF OMITTED] TN31DE97.013
The MSHA and NIOSH joint finding establishes that for valid
measurements made with an approved sampler unit, CVtotal is
in fact less than CVCTV at all dust concentrations
(i).
The situation in which measurement error is most likely to cause an
erroneous noncompliance determination is the hypothetical case of
i = S for either a single, full-shift measurement
or for all of the measurements made in the same MMU. In that borderline
situation--i.e., the worst case consistent with Ho--the
standard deviation is identical for all measurement errors. Therefore,
the value of s used in constructing the CTV table is the product of S
and CVCTV evaluated for a dust concentration equal to S:
[GRAPHIC] [TIFF OMITTED] TN31DE97.014
Assuming a normal distribution of measurement errors as explained
above, it follows that the probability a single measurement would equal
or exceed the critical value
[GRAPHIC] [TIFF OMITTED] TN31DE97.015
is five percent under Ho when CVtotal =
CVCTV. The tabled CTV corresponding to S is derived by
simply raising the critical value c up to the next exact multiple of
0.01 mg/m3.
For example, at a dust concentration (i) just
meeting the applicable standard of S = 2 mg/m3,
CVCTV is 9.95 percent. Therefore, the calculated value of c
is 2.326 and the CTV is 2.33 mg/m\3\. Any valid single, full-shift
measurement at or above this CTV is unlikely to be this large simply
because of measurement error. Therefore, any such measurement warrants
a noncompliance citation.
The probability that a measurement exceeds the CTV is even smaller
if i>S for any I. Furthermore, to the extent that
CVtotal is actually less than CVCTV, is
actually less than SCVCTV. This results in an even
lower probability that the critical value would be exceeded under the
null hypothesis. Consequently, if any single, full-shift measurement
equals or exceeds c, then Ho can be rejected at confidence
level of at least 95-percent. Since rejection of Ho implies
that i S for at least one value of I,
this warrants a noncompliance citation.
It should be noted that when each of several measurements is
separately compared to the CTV table, the probability that at least one
i will be large enough to force Xi
CTV when S is
greater than the probability when only a single comparison is made. For
example (still assuming S = 2 mg/m3), if CVtotal
is actually 6.6%, then the standard deviation of
is 6.6% of 2.0 mg/m3, or 0.132
mg/m3, when = S. Using
properties of the normal distribution, the probability that any single
measurement would exceed the CTV in this borderline situation is
calculated to be 0.0062. However, the
[[Page 5701]]
probability that at least one of five such measurements results in a
citation is 1--(0.9938)5 = 3.1 percent. Therefore, the
confidence level at which a citation can be issued, based on the
maximum of five measurements made in the same MMU on a given shift, is
97%.
The constant 1.64 used in calculating the CTV is a 1-tailed 95-
percent confidence coefficient and is derived from the standard normal
probability distribution. At least one commenter expressed confusion
about whether the CTV table is based on a 1-tailed or a 2-tailed
confidence coefficient. This commenter claimed that MSHA's use of a
confidence coefficient equal to 1.64 ``clearly establishes a 90%
confidence level'' rather than 95%. The commenter apparently confused
the CTV for rejecting a 1-tailed hypothesis
( S) with the pair of critical
values for rejecting a 2-tailed hypothesis
( = S) and inferring that
i simply differs from S in either direction. The
criterion for rejecting the latter hypothesis would be a measurement
either sufficiently above the applicable standard or sufficiently below
it. In testing for a difference of arbitrary direction, 1.64 would
indeed yield a pair of 90-percent confidence limits, with a 5-percent
chance of erring on either side. The purpose of the CTV table, however,
is to provide criteria for determining that the true dust concentration
strictly exceeds the applicable standard. Since such a determination
can occur only when a single, full-shift measurement is sufficiently
high, there is exactly zero probability of erroneously citing
noncompliance when a measurement falls below the lower confidence
limit. Consequently, the total probability of erroneously citing
noncompliance equals the probability that a standard normal random
variable exceeds 1.64, which is 5 percent.
One commenter alluded to testimony in the Keystone case (Keystone
v. Secretary of Labor, 16 FMSHRC 6 (Jan. 4, 1994)), suggesting that
application of the CTV to a single measurement involves an invalid
comparison of two distributions or comparison of two means. Contrary to
much of the testimony presented in that case, a determination of
noncompliance using the CTV table is based on the decision procedure
described above. It does not involve any comparison of probability
distributions or means. Nor does it involve any statistical
distribution of dust concentrations. It involves only the comparison of
an individual full-shift measurement to the applicable standard. There
is only one probability distribution involved in this comparison:
namely, the distribution of random measurement errors by which each
full-shift measurement deviates from the true dust concentration to
which the sampler unit is exposed.
Some commenters apparently misunderstood the effect of potential
weighing errors on the formula for calculating the CTV corresponding to
different applicable standards. Weight gain is estimated from the
difference between two weighings of an exposed filter capsule, adjusted
by subtracting the difference between two weighings of a control filter
capsule. Since weight gains are small compared to the total weight of
capsules being weighed, any dependence of weighing error on the
magnitude of the mass being weighed is canceled in the process of
calculating the difference. Since the standard deviation of the error
in weight gain is, therefore, essentially constant, the ratio of that
standard deviation to the dust concentration being measured decreases
with increasing dust concentration. This causes CVCTV to
decrease as the dust concentration increases. As explained above, the
CTV corresponding to S is calculated using the value of
CVCTV for dust concentrations exactly equal to S.
Consequently, the CTV corresponding to a standard of 2.0 mg/
m3 is based on a smaller value of CVCTV than the
CTV corresponding to a standard of 0.2 mg/m3.
One commenter implied that use of the CTV table relies on an
assumption that CVtotal declines at concentrations greater
than 2.0 mg/m3 (or S in general). As explained previously,
the CTV corresponding to different applicable standards is designed to
test the null hypothesis that S is not exceeded. For each applicable
standard, entries are based on the probability distribution of
observations expected under that presumption. Consequently, the
magnitude of CVtotal assumed in establishing or applying any
CTV does not decrease below the value of CVtotal calculated
for a concentration of 2.0 mg/m3, since that is the maximum
applicable standard being tested. Because the probability of wrongly
citing noncompliance is zero when S is exceeded, measurement
uncertainty at concentrations greater than S is not relevant to
noncompliance determinations. (It would, however, be relevant to
inferring compliance at a specified confidence level--i.e., to a test
of the alternative hypothesis that S is not exceeded.)
III. Validity of the CTV table
Some commenters questioned the validity of the CTV table and
challenged the formula used to calculate each CTV listed. Some objected
to the use of a normal distribution and claimed that a lognormal
distribution or nonparametric assumptions would be more appropriate.
Other commenters objected specifically to the use of a confidence
coefficient based on a standard normal probability distribution, rather
than a t-distribution. The validity of using n, rather than
(n-1), in the formula used to calculate citation threshold
values in MSHA's February 1994 notice, was also questioned. At least
one commenter contended that the formula used to generate the CTV table
is not valid for use with only one measurement.
Such comments would have some validity if the CTV table were
intended to test or estimate average concentration over some spatially
distributed region of a mine or some period greater than the single
shift during which each measurement is taken. In either case, it might
be necessary and appropriate to estimate variation in concentration
directly from the measurement samples obtained. Such an estimate could
conceivably be used in establishing a site-specific threshold value for
citation. This would, indeed, require a theoretical minimum of two
samples, or far more for valid practical applications. Estimating
variability from the samples collected would also require additional
assumptions or nonparametric methods to reflect the pattern of
variation in dust concentration between locations or shifts.
The objections raised, however, apply to a very different task from
the one for which the CTV table is designed. As explained previously,
the CTV table is not meant to test dust concentration averaged over any
period greater than the shift during which measurements were taken. Nor
is it meant to test dust concentration averaged across different
occupational locations or throughout any spatially distributed region
of the mine. Instead, the CTV table provides criteria for determining
noncompliance at individual sampling locations on individual shifts.
Neither the spatial nor temporal distribution of the dust
concentrations is germane to the intended citation criteria. Although
several measurements may be taken during a single inspection, MSHA
regards each of these measurements as relating to the dust
concentration uniquely associated on a given shift with a separate
sampling location. Each such dust concentration (i)
is the average for the atmosphere at the sampling location, accumulated
over the course of the single, full shift sampled. Since the
enforcement objective is to determine whether i > S
for any individual I, it is not necessary to estimate or assume
anything about the
[[Page 5702]]
degree to which i varies from location to location
or from shift to shift. Nor is it necessary to assume anything about
the spatial or temporal statistical distribution of
i. No such assumptions are built into the CTV
table. A normal distribution is imputed only to
, the difference between Xi and
i. Since the mean across various
i is not being estimated or tested, it is not
necessary to estimate variability among the i from
measurements taken during the inspection. MSHA emphatically agrees with
those commenters who stressed the impossibility of doing so with a
single measurement.
Those commenters who objected to MSHA's use of a normal
distribution, claiming that a lognormal distribution or nonparametric
assumptions would be more appropriate, apparently confused the
distribution of dust concentrations over time and between locations
with the distribution of errors that arise when measuring dust
concentration at a specific time and location. In other words, they
confused the distribution of i with the
distribution of . The concerns about non-
normality stem from confusion about what quantity is being estimated.
MSHA does not dispute the fact that lognormal or nonparametric
methods are often appropriate for modeling variability in occupational
dust concentrations. MSHA, however, is explicitly not claiming to
estimate any quantity beyond the average dust concentration at a
particular sampling location on a single shift. MSHA does not claim
that dust concentrations are normally distributed from shift to shift,
from occupation to occupation, or from location to location; nor is any
such assumption built into the CTV table. Since the object is not to
estimate average concentration over a range of different locations or
shifts, the statistical distribution of i is
irrelevant, and application of lognormal or nonparametric techniques in
constructing citation criteria is both unnecessary and inappropriate.
In constructing the CTV table, MSHA used a normal probability
distribution solely to represent a potential measurement error,
. This measurement error causes a
measurement Xi to deviate from i, the
actual dust concentration at a specific time and place. As
distinguished from the statistical distribution of dust concentrations,
it is generally accepted that the distribution of measurement errors
around a given concentration is normal [3]. This was explicitly
acknowledged by members of the industry panel in their Morgantown
testimony.
Similarly, criticism directed against MSHA's use of a confidence
coefficient derived from the standard normal distribution instead of
the t-distribution arises from a basic misunder standing of what is or
is not being estimated in the decision procedure. Contrary to the
remark of one commenter, use of the t-distribution is not justified as
a ``compromise'' between normal-theoretic and nonparametric
assumptions. The
t-distribution arises in statistical theory when a normally distributed
random variable is divided by an estimate of its standard deviation.
Typically it is applied to situations in which the mean and standard
deviation are estimated from the same normally distributed data,
consisting of fewer than about thirty or forty random data points. If
the estimate of standard deviation is based on more data, then the
confidence coefficient derived from the t-distribution is approximately
equal to the corresponding value derived from the standard normal
distribution. Use of the t-distribution is appropriate, for example,
when a group of normally distributed observations is ``standardized''
by subtracting the group mean from each observation and dividing the
result by the group standard deviation.
Those commenters advocating a confidence coefficient based on the
t-distribution failed to recognize that CVCTV was not
derived from the measurements that MSHA inspectors will use to test for
compliance with S. Use of the t-distribution is not appropriate when an
independently known or stipulated standard deviation is used in
comparing observations to a standard [3]. The standard deviation of
measurement errors used in constructing the CTV table is derived from
prior knowledge, rather than estimated from a few measurements taken
during an inspection. Experimental analysis has shown that
CVtotal is less than CVCTV. So long as this is
true, use of a confidence coefficient derived from the standard normal
distribution is entirely appropriate.
Contrary to the claims of some commenters, there is no valid basis
for including a so-called [n/(n-1)]1/2 ``correction factor''
in the formula for establishing a CTV. (The ``n'' in this expression
would refer to the number of measurements, if a noncompliance
determination were based on the average of several measurements.) The
theory behind such a factor does not apply when, as in the case of the
CTV table, a predetermined or maximum tolerated variability in
measurement error is used in comparing observations to a standard [3].
It would apply only if variability in measurements observed during each
inspection were somehow used to construct a CTV specific to that
inspection. The variability observed among multiple samples collected
during an MSHA inspection has little to do with the accuracy of an
individual measurement and is not used at all in constructing the CTV
table.
Although no explicit reason was given for the claim by some
commenters that the formula used to generate the CTV table is not valid
for use with a single measurement, this would follow if either: (1) the
appropriate basis for the confidence coefficient were a
t-distribution rather than a standard normal distribution; or (2) it
were necessary to multiply the CTV by [n/(n-1)]1/2, where n
is the number of measurements on which a noncompliance determination is
based. In the former case, the standard normal distribution would not
adequately approximate the t-distribution; and in the latter case, n =
1 would cause the so-called correction factor, and hence the CTV, to be
mathematically indeterminate for determinations based on a single
sample. It has already been explained, however, that neither of these
considerations are applicable to the CTV table.
Some commenters stated that a single measurement cannot accurately
be used to detect excessive dust concentrations, even if the
noncompliance determination applies only to a specific shift and
location. These commenters implied that due to random, temporary
fluctuations in dust concentration, a single measurement is inherently
unstable and misleading. Such arguments fail to differentiate a full-
shift sample from a ``grab sample,'' which is typically a sample
collected over only a few minutes or seconds and used to estimate
average conditions over an entire shift. In contrast to a grab sample,
each full-shift dust sample is collected continuously over the full
period to which the measurement applies. An 8-hour dust sample consists
of 480 1-minute grab samples, or an arbitrarily large number of even
shorter grab samples. A full-shift dust sample can be viewed as
measuring average concentration over the entire shift by averaging
together all of these shorter subsamples. Although short-term
fluctuations in dust concentration, as well as random changes in flow
rate and collection efficiency, may cause many of the subsamples to
poorly represent average concentration over the entire shift, random
short-term aberrations tend to cancel one another when the subsamples
are combined. Therefore, a
[[Page 5703]]
full-shift dust sample does not suffer from lack of sample size.
Appendix C--Risk of Erroneous Enforcement Determinations
I. What Constitutes Compliance or Noncompliance?
To simplify the following discussion, let denote the
average dust concentration to which a sampler unit is exposed on a
given shift, let S denote the applicable standard, and let X denote a
valid, full-shift measurement of . Also, let c be the CTV in
the table corresponding to S so that a citation is issued when X
c. Section 202(b)(2) of the Mine Act requires that the
average dust concentration during each shift be maintained at or below
the applicable standard wherever miners normally work or travel. This
means that, on any given shift, the average dust concentration
() at any valid sampling location must not exceed the
applicable standard (S).
Since the CTVs listed always exceed S it can happen that a full-
shift measurement (X) falls between S and c. In such instances, MSHA
will not issue a citation. This does not, however, imply that MSHA
considers the mine atmosphere sampled to have been in compliance with
the Mine Act or that cases of marginal noncompliance are tolerable.
MSHA's use of the CTVs is not motivated by any tacit acceptance of
marginal noncompliance. Rather, it is motivated by the necessity to
avoid unsustainable violations. When X falls between S and c, this
provides some evidence that > S; but the evidence is
insufficient to warrant a citation.
Although > S constitutes a violation, X greater than S
but less than the CTV does not provide compelling evidence that
> S. This is because, in a sufficiently well-controlled
mining environment, X is more likely to slightly exceed S due to
measurement error than due to > S. In fact, as demonstrated
in Appendix D, citing when X > S but X < c="" could="" result="" in="" citations="" when="" the="" probability="" of="" compliance=""> S) on the
shift and location sampled is greater than 50 percent. Use of the CTV
table is necessary in order to avoid citing in such cases.
There are two sorts of conclusions that might be drawn from the
results of a single MSHA inspection: those relating to the individual
shift sampled and those relating to some longer time period, such as
the full interval between MSHA inspections. Therefore, in evaluating
the probability of erroneous enforcement determinations, it is
essential to distinguish between (1) compliance or noncompliance with
the applicable standard on the shift sampled and (2) compliance or
noncompliance with the full requirement of the Mine Act as it applies
to every shift over a longer term, such as the period between MSHA
inspections.
If > S on some proportion of shifts, say P < 1,="" then="" the="" mine="" does="" not="" comply="" with="" the="" applicable="" standard="" on="" some="" individual="" shifts="" and,="" therefore,="" does="" not="" comply="" with="" the="" mine="" act="" over="" the="" longer="" term.="" at="" the="" same="" time,="" the="" mine="" is="" in="" compliance="" with="" the="" applicable="" standard="" (at="" the="" location="" sampled)="" on="" a="" complementary="" proportion,="" equal="" to="" 1--p,="" of="" individual="" shifts.="" if="" an="" msha="" inspection="" happens="" to="" fall="" on="" one="" of="" those="" shifts="" that="" is="" out="" of="" compliance,="" then="" a="" correct="" determination="" with="" respect="" to="" the="" individual="" shift="" would="" also="" be="" correct="" with="" respect="" to="" the="" longer="" term.="" if,="" on="" the="" other="" hand,="" the="" msha="" inspection="" happens="" to="" fall="" on="" a="" shift="" that="" is="" in="" compliance,="" then="" it="" would="" be="" a="" mistake="" to="" assume="" compliance="" on="" subsequent="" shifts="" and="" vice="" versa.="" although="" msha="" interprets="" the="" mine="" act="" as="" requiring=""> S on each shift and at each sampling location to
which miners in the active workings are exposed, the immediate
objective of an MSHA dust inspection can only be to determine
compliance or noncompliance for the shift and location sampled.
Therefore, MSHA does not consider a compliance or noncompliance
determination to be erroneous if it is correct with respect to the
individual shift and location but incorrect with respect to other
shifts or locations.
II. Uncertainty in the Standard-Setting Process
In response to the March, 12, 1996 MSHA/NIOSH Federal Register
notice, a commenter claimed that a noncompliance determination based on
a single, full-shift measurement could be erroneous if the applicable
standard was improperly established due to measurement errors
associated with silica analysis. It was, therefore, suggested that
uncertainty in the standard-setting process should be factored into the
risk of erroneous enforcement decisions. MSHA agrees that, like any
measurement process, the sampling and analytical method used to
quantify the silica content of a respirable dust sample in order to set
the applicable standard is subject to potential measurement errors.
Therefore, MSHA uses an analytical procedure that meets the requirement
of a NIOSH Class B analytical method. Applicable standards are set
based on results of silica analysis using the most up-to-date
laboratory equipment.
The Secretary, however, considers the accuracy of the standard-
setting process to be a separate issue from the accuracy of
noncompliance determinations based on a single-full-shift measurement,
once the applicable standard has been set. The present notice relates
only to the enforcement of the applicable standard in effect at time of
the sampling inspection. Therefore, the following discussion treats any
applicable standard in effect at the time of sampling as binding and
evaluates the risk of erroneous determinations relative to that
standard.
III. Measurement Uncertainty and Dust Concentration Variability
Variability in dust concentration refers to the differing values of
on different shifts or at different locations. For a given
value of , measurement uncertainty refers to the differing
measurement results that could arise because of different potential
measurement errors. If S, measurement error can
cause an erroneous citation. Similarly, if > S, then
measurement error can cause an erroneous failure to cite.
The ``margin of error'' separating each CTV from the corresponding
applicable standard does not eliminate the possibility of erroneous
enforcement determinations due to uncertainty in the measurement
process. A determination based on comparing X to the CTV could be
erroneous in either of two ways with respect to the individual shift
sampled: (1) the comparison could erroneously indicate noncompliance on
the shift (i.e, X c but S) or (2) the
comparison could erroneously fail to indicate noncompliance on the
shift (i.e, X < c="" but=""> > S). The margin of error built into
the CTV table reduces the probability of erroneous citations but
increases the probability of erroneous failures to cite.
MSHA recognizes that in determining how large the margin of error
should be, there is a tradeoff between the probabilities of these two
mistakes--i.e., if the chance of erroneously failing to cite is
reduced, then the chance of erroneously citing is increased, and vice
versa. MSHA has constructed the CTV. table so as to ensure that
citations will be issued only when they can be issued at a high level
of confidence. As will be shown below, doing this provides assurance
that for any given citation, is more likely than not to
actually exceed S. In contrast, if there were no margin of error,
citations more likely than not to be erroneous could occasionally be
issued. Examples of this are given in Appendix D.
[[Page 5704]]
In the discussion below, the risk of erroneous citations and
erroneous failures to cite is quantified for noncompliance
determinations based on the CTV table. To illustrate points in the
theoretical discussion, three different mining environments will be
used as examples. These environments exemplify different degrees of
dust concentration variability and dust control effectiveness. The
first example (Case 1) is based on historical mine data provided by
commenters in connection with these proceedings. The second and third
examples (Case 2 and Case 3) are hypothetical and are designed to
reflect extremely well-controlled and poorly controlled mining
environments, respectively. In these three examples, it will be assumed
that is lognormally distributed from shift to shift. This is
a standard assumption for airborne contaminants in an occupational
setting [3]. The three cases considered are characterized as follows:
----------------------------------------------------------------------------------------------------------------
Dust concentration (mg/m3)
-----------------------------------------------------------------
Case Arithmetic Arithmetic Prb {>S}
E{} SD{} mean Std. Dev. (percent)
----------------------------------------------------------------------------------------------------------------
1............................................. 1.66 0.70 1.53 1.50 25.4
2............................................. 1.20 0.24 1.18 1.22 0.4
3............................................. 2.20 1.32 1.89 1.74 45.8
----------------------------------------------------------------------------------------------------------------
In addition to the variability in dust concentrations described by
the arithmetic and geometric standard deviations of , full-
shift measurements contain a degree of uncertainty described by
CVtotal, the coefficient of variation for measurements of
the same dust concentration. In calculating the probability of
erroneous determinations for the three example cases, it will also be
assumed that the applicable standard is S = 2.0 mg/m3 and
that the coefficient of variation in full-shift measurements taken at a
given value of is:
[GRAPHIC] [TIFF OMITTED] TN31DE97.016
Where e = 9.12 g is the standard deviation
of error in weight gain, as determined from MSHA's 1995 field
investigation of measurement precision [4]; 1.38 is the MRE-equivalent
conversion factor for measurements made with an approved sampler unit;
the first quantity being squared is CVweight;
CVpump = 4.2% and CVsampler = 5%, as explained in
Appendix B.II of the joint MSHA and NIOSH notice of finding published
elsewhere in today's Federal Register.
It should be noted that the ``total'' in CVtotal refers
to total measurement uncertainty and is not meant to include the
effects of variability in dust concentration.
Because it employs a higher value for CVsampler
(reflecting variability amongst used rather than new 10-mm nylon
cyclones), this composite estimate of CVtotal is slightly
greater and perhaps slightly more realistic than that obtained directly
from MSHA's 1995 field investigation. It declines from 11.3% at dust
concentrations of 0.2 mg/m3 to no more than 6.6% at
concentrations of 2.0 mg/m3 or greater. At all dust
concentrations within this range, it falls well below the 12.8% maximum
value permitted for a method meeting the NIOSH Accuracy Criterion [5].
It is also smaller than the value, CVCTV, used to construct
the CTV table. As explained in Appendix B, this ensures that any
citation issued will be warranted at a confidence level of at least 95
percent.
To simplify the discussion below on risk of erroneous citations and
erroneous failures to cite, it is necessary to introduce some
additional notation and to focus on just one measurement collected
during each inspection.2 This could be the ``D.O.'' sample
in a MMU, or the measurement collected for a designated area. Let
= X- represent the measurement error in a valid
measurement. For reasons explained in Appendix B, is assumed
to be normally distributed with zero mean and standard deviation equal
to = CVtotal.
Consequently, X is normally distributed with mean equal to
and standard deviation equal to . This normal distribution of
X around reflects uncertainty in the measurement of a given
dust concentration. On any given shift, the probability distribution of
X is determined by the value of for that shift and sampling
location. Therefore, the probability of citation on a given shift is
conditional on and is denoted by Prb{Xc |
.}3
---------------------------------------------------------------------------
\2\ Appendix D addresses cases in which a noncompliance
determination is based on the maximum of several measurements.
\3\ A vertical bar is used to denote conditional probability.
Prb {A | B} denotes the conditional probability of event A, given
the occurrence of event B. For any events A and B,
Prb{A|B}=Prb{A and B}/Prb {B}=Prb{B|A}Prb {A}/Prb{B}
---------------------------------------------------------------------------
Since varies from shift to shift, variability in dust
concentration is represented by the probability distribution of
. Let E {} denote the expected (i.e., arithmetic
mean) dust concentration over some longer term of interest, such as the
interval between MSHA inspections; and let SD{} denote the
standard deviation of over the same period. Although the
value of on any individual shift is unknown,
Prb{Xc} can be calculated using the probability distribution
of . In particular, if the probability is known that
fulfills a specified condition, such as S or
> S, then
Prb{Xc} = Prb{Xc | S}
Prb{S}+Prb{Xc |
>S}
Prb{>S}.
Over a sufficiently long term, with respect to any particular
sampling
[[Page 5705]]
location, Prb{>S} and Prb{S} can be
identified, respectively, with the proportion of noncompliant shifts,
P, and the proportion of compliant shifts, 1-P. P is sometimes called
the noncompliance fraction and more or less defines the likelihood that
the applicable standard is or is not exceeded on the particular shift
inspected.4
---------------------------------------------------------------------------
\4\ P defines this likelihood exactly only if shifts are
randomly selected for MSHA inspection and there is no adjustment of
conditions in response to the inspection.
---------------------------------------------------------------------------
If the statistical distribution of can be adequately
represented by a probability density function, denoted f(),
then Prb{>S} and Prb{S} can also be
calculated by integrating f() over the desired range. The
probability that falls in any interval, say between a and b,
is given by:
[GRAPHIC] [TIFF OMITTED] TN31DE97.017
It follows that:
[GRAPHIC] [TIFF OMITTED] TN31DE97.018
IV. Risk of Erroneous Citation
Some commenters argued that a citation for noncompliance is
warranted only if the average dust concentration to which a miner is
exposed exceeds the applicable standard over a period of time greater
than a single shift, such as a bimonthly sampling period, a year, or a
miner's lifetime. Therefore, these commenters called it ``unfair'' to
cite individual shifts on which the applicable standard is exceeded, so
long as the average over this longer term meets the applicable
standard. For example, based on the historical sampling data provided
by a commenter and employed here as Case 1, one commenter concluded
that ``* * * there is at least a 1 in 6 or 17% probability that any
single sample can show potential overexposure [using the CTV table]
when one does not exist.'' Further, these commenters maintained that
basing citations on a single, full-shift measurement would
substantially increase the frequency of unfair citations, compared to
existing MSHA policy.
Using the notation introduced above, these commenters have confused
with E() and confounded the noncompliance fraction P
with the probability of erroneous citation. For example, the 17-percent
figure mentioned above includes all cases in which X c,
regardless of whether > S on the shift sampled. In the
discussion accompanying the data, commenters argue that since
E() is approximately 1.66 mg/m\3\, or less than 1.85 mg/m\3\
at a high confidence level, ``* * * [cases of X c] show
potential overexposure when one does not exist.'' This statement
depends on the unwarranted assumption that miners exposed to these
conditions have been exposed to similarly distributed dust
concentrations in the past and that they will be exposed to similarly
distributed concentrations in the future. These commenters' own
analysis indicates that the dust concentration has not been kept below
the standard on each shift. Therefore, a citation is warranted under
the Mine Act.
To more fully explore what is going on in Case 1, suppose, as these
commenters suggest, that dust concentrations over the period observed
are lognormally distributed from shift to shift, with E{} =
1.66 mg/m3 and a geometric standard deviation of about 1.5
mg/m3. Under this assumption, > 2.0 mg/
m3 on more than 25 percent of all shifts, and >
2.33 mg/m3 on 15 percent. These percentages pertain to
actual dust concentrations and have nothing to do with measurement
error or accuracy of an individual measurement. Therefore, a 2.0 mg/
m3 dust standard would be violated on 25 percent of all
production shifts. The applicable standard would be violated by an
amount greater than 0.33 mg/m3 on 15 percent. Since 2.33 is
the CTV for a single measurement, this 15 percent actually represents
shifts sufficiently far out of compliance that they would probably be
cited if inspected. Nevertheless, the commenters' analysis includes
such shifts in the 17 percent claimed as cases subject to erroneous or
unfair citation.
The expected value of the noncompliance fraction (P) in Case 1 is
25 percent. Therefore, close to 25 percent of all single shift
measurements made under the conditions of Case 1 would be expected to
exceed the standard. Only 17 percent of the single full-shift
measurements taken, however, exceeded the CTV and would have warranted
citations. Using the estimate of CVtotal described above, 15
percent of all single shift measurements would be expected to do so.
Therefore, contrary to the commenters' conclusion, Case 1 does not
demonstrate a high probability of erroneously identifying
overexposures. Instead, it illustrates an effect of the high confidence
level required for citation: the margin of error built into the CTV
reduces the probability of citing whatever shift happens to be selected
for inspection from about 25 percent to 15 percent. Although the
applicable standard is violated on 25 percent of the shifts, there is
only a 15 percent chance that any particular measurement meets the
citation criterion.
To correctly and unambiguously quantify the risk of ``unfair''
citations, it is necessary to identify three distinct ways of
interpreting the risk of erroneous noncompliance determinations. This
risk can be defined alternatively as:
(1) the probability of citing when the mine atmosphere sampled is
actually in compliance, Prb{Xc|S};
(2) the probability that the mine atmosphere on a shift randomly
selected for inspection is in compliance but is nevertheless cited,
Prb{S and Xc}; or
(3) the probability that a given citation is erroneous,
Prb{S|Xc}.
These three different probabilities apply to three different base
populations. Although the different interpretations of risk give rise
to quantitatively different probabilities, the expected total number of
erroneous citations, denoted N, remains constant if each
probability is multiplied by the size of the population to which it
applies. To obtain N, the first probability must be multiplied
by the number of valid measurements made when S,
the second by the total number of valid measurements, and the third by
the total number of citations issued--i.e., valid measurements for
which X c.
The CTV table limits the probability of erroneously citing defined
by the first two interpretations to a maximum of less than five
percent. However, in a
[[Page 5706]]
well-controlled mining environment, where citations are rarely
warranted, the third probability can be larger than the first two.
Since the burden of proof rests with MSHA to demonstrate noncompliance,
it is essential that deg. be kept well below 50 percent. As
will be shown by example, the use of the CTV table accomplishes this
goal.
Each of the three different probabilities related to erroneous
noncompliance determinations will now be explained in detail.
Calculations for all examples are performed under the assumptions (1)
that is lognormally distributed and (2) that is
normally distributed with mean equal to zero and standard deviation
equal to CVtotal.
1. = Prb{Xc|S}
The first risk to be considered is the probability of citing
noncompliance when the mine atmosphere sampled is actually in
compliance. This probability represents the proportion of those
measurements made when S that result in X
c. In other words,
=Prb{Xc|S} is the probability
that, due to measurement error, a citation is issued under the
condition that S. This is the probability
associated with what is commonly designated Type I error for testing
the null hypothesis: S on the shift sampled.
Essentially, is the expected (i.e., mean) probability of
citation over all those shifts sampled that are at or below the
applicable standard. The relative frequency distribution of
over those shifts is described by its probability density function,
f(). Therefore, can be calculated as follows:
[GRAPHIC] [TIFF OMITTED] TN31DE97.019
If did not vary, then would be directly related
to the confidence level at which the null hypothesis could be rejected
when X c. That confidence level, which applies to citations
issued in accordance with the CTV table, is defined as the minimum
possible value of 1-Prb{Xc|}, subject to the
restriction that S. There is a subtle but
extremely important distinction between this and 1-. Among all
those shifts on which S,
Prb{Xc|} is maximized when = S.
Therefore, the minimum possible value of 1-, arises when
= S on every shift. The resulting confidence level for
concluding > S when X c is equal to
1-Prb{Xc|=S}. For the value of CVtotal
described above (i.e., 6.6% when = S = 2.0 mg/m3),
this works out to a confidence level of 0.99, or 99%.
Although MSHA interprets the Mine Act as requiring
S on each shift at any location to which a miner in the
active workings is exposed, citations for noncompliance are intended to
apply only to the shift and location sampled. Therefore, MSHA makes no
assumption regarding the relative frequency distribution of
from shift to shift. This is consistent with the concept of defining
the confidence level according to the scenario most susceptible to an
erroneous determination under the null hypothesis. However, the
resulting confidence level for citing when X c really
applies only to the hypothetical case most susceptible to erroneous
citation.
In reality, so long as falls below S on some shifts,
will be smaller than 0.01. The further falls below
the applicable standard, and the more shifts on which this occurs, the
less likely it becomes that measurement error alone () will be
great enough to cause X c on a shift randomly selected for
inspection. For example, if S = 2.0 mg/m3, then c = 2.33 mg/
m \3\.
Therefore, if = 1.8 mg/m3, a citation would be
issued only if c-. An
0.53 mg/m3 (resulting in X 2.33 mg/
m3) amounts to a measurement error greater than 29 percent
of the true dust concentration. If the sample is valid, then the
probability of such an occurrence (given that CVtotal = 6.6%
at = 1.8 mg/m3) is less than 4 per million. This
illustrates the general point that Prb{Xc|} can be
far less than 0.01 when < s.="" since="">c|} is smaller the further
falls below S, Prb{Xc|S} depends on the
probability distribution of