Page:NIOSH DM DFM respirator evaluation draft.pdf/49

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WORKING DRAFT 9.15.92—Performance Evaluation of DM and DFM Filter Respirators
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  The computation of a tolerance limit enables one to create an interval estimate for the range of values around the point estimate within which we are confident (at a specified confidence level) that the actual 5th-percentile WPF lies. The interval estimate defines the error band for the actual 5th-percentile WPF. It is similar to the margin of error typically reported with the results of public opinion polls.
  A 1 -sided lower tolerance limit computed at the 95% confidence level for the 5th-percentile WPF would be denoted as LTL1,96,06. A 1-sided lower tolerance limit is & type of confidence limit below which we expect a stated proportion of a population to

lie.[1] With a tolerance limit one can then assess the amount of uncertainty or mar- gin of sampling error associated with a point estimate of the actual 5th-percentile WPF.
  Respirator researchers may conclude that their WPF data substantiate an APF of 10 for non-powered, air-purifying halfmasks if their point estimate for their 5th-percentile WPF exceeds 10. However, this approach to reaching research conclusions does not consider the uncertainty in their point estimate due to sampling errors. By not computing a 1-sided lower tolerance limit for their actual 5th-percentile WPF, they may reach erroneous conclusions regarding their study results, since the actual 5th-percentile WPF may be lower than the point estimate for this value. In this case the actual proportion of wearers expected to exceed the 5th-percentile WPF point estimate would exceed 5%.
  For example, Lenhart and Campbell studied the performance of a non-powered, HEPA-equipped halfmask on 25 test subjects.[2] For their data they reported a 5th-percentile WPF point estimate of 18 and concluded that the use of an APF of 10 "for the negative pressure halfmask is not discredited" and "an assigned protection factor of 10 is appropriate for the half-mask negative pressure air-purifying respirator evaluated in this study."[3] They also computed a 1-sided lower tolerance limit for the actual 5th-percentile WPF and stated that "at a confidence level of 90% (y = 0.9) approximately 95% (P = 0.95) of the negative pressure respirator workplace protection factors exceed a value of 10."[4]

  1. ↑ Leidel, N. A. and K. A. Busch: Statistical Design and Data Analysis Requirements. Chapter 8 of Patty's Industrial Hygiene and Toxicology, Volume III, Theory and Rationale of Industrial Hygiene Practice, Second Edition, Volume 3A, The Work Environment, Cralley, L. J. and L. V. Cralley, Editore, John Wiley & Sons, Inc., New York, (1986), Sections 6.7 and 6.13.
  2. ↑ Lenhart, S.W. and D. L. Campbell: Assigned Protection Factors for Two Respirator Types Based Upon Workplace Performance Testing, Ann. Occup. Hyg. 28(2):173-182 (1984).
  3. ↑ Ibid., pp. 180-181.
  4. ↑ Ibid, p. 181.
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