analyzing small data sets

Transcription

1 Review of Lognormal Statistics and Review of Lognormal Statistics and analyzing small data sets

2 Review of IH Statistics I. Lognormal distribution II. III. IV. Sample 95 th percentile UCL for the sample 95 th percentile Rules-of-thumb for Eyeballing Exposure Data 2

3 I. Lognormal Distribution Example Airborne exposures to inorganic lead source: Cope et al. AIHAJ 40: ,

4 Copyright 2007 Exposure Assessment Solutions, Inc. 4

5 Parameters vs. Statistics Parameters Statistics -calculated using all elements of the population -log transform each element -calculated from a sample of n elements randomly selected -log transform each element Population Mean μ y Sample Mean _ y Population Standard Deviation σ y Sample Standard Deviation s y The measurements are converted to natural logs: y = ln(x) 5

6 Parameters vs. Statistics Parameters -calculated using all elements of the population Statistics -calculated from a sample of n elements randomly selected Population Geometric Mean GM Sample Geometric Mean gm Population Geometric Standard Deviation GSD Sample Geometric Standard Deviation gsd 6

7 Lognormal distribution PDF GM 7

8 Lognormal 8

9 Sample geometric mean (gm) & geometric standard deviation (gsd) 9

10 Example: Welding fume data - estimate GM and GSD Case x i (mg/m 3 ) y i =ln(x i ) (y i -y) Sum = _ y = gm = _ gsd =

11 Example: Welding fume data - estimate GM and GSD 11

12 Example: Welding fume data - estimate μ and σ Case x i (mg/m 3 ) ( x i -x ) Sum = _ x = _ sd =

13 Example: Welding fume data - estimate μ and σ 13

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15 GSD = X84/X50 = 31/12= 3.1/

16 II. Sample 95 th Percentile Exposure The focus is on the upper tail of the exposure profile. The sample 95 th percentile can be considered a decision statistic. The (usual) goal is to determine which category the 95 th Percentile most likely l falls. It is used to assist in reaching a decision that the exposure profile is Controlled or Acceptable Unacceptable or falls in a Control Category 16

17 95 th Percentile interpretation of TWA OELs ACGIH Roach, S.A., Baier, E.J., Ayer, H.E., and Harris, R.L.: Testing compliance with Threshold Limit Values for respirable dusts. American Industrial Hygiene Association Journal 28: (1967). Stokinger, H.E.: Industrial air standards - theory and practice. Journal of Occupational Medicine 15: (1973). Still, K.R. and Wells, B.: Quantitative Industrial Hygiene Programs: Workplace Monitoring. (Industrial Hygiene Program Management series, part VIII). Applied Industrial Hygiene 4:F14-F17 (1989). 17

18 95 th Percentile interpretation of TWA OELs AIHA 1991 and 1998 guidance Employer should maintain true group or individual upper percentile exposure < TWA OEL Similar Exposure Group 95 th percentile exposure < TWA OEL Corn, M. and Esmen, N.A.: Workplace exposure zones for classification of employee exposures to physical and chemical agents. American Industrial Hygiene Association Journal 40:47-57 (1979). 18

19 95 th Percentile interpretation of TWA OELs NIOSH guidance Employer should 95% confident that 95% of the exposures are < the TWA PEL Leidel, N.A., Busch, K.A., Lynch, J.R.: Occupational Exposure Sampling Strategy Manual. National Institute for Occupational Safety and Health (NIOSH) Publication No (available as a pdf file from NIOSH website) (1977). OSHA Measured TWA exposures should rarely exceed the TWA PEL (preamble to the benzene PEL, 1987) 19

20 95 th Percentile interpretation of TWA OELs EU CEN (Comité Européen de Normalisation): Workplace atmospheres - Guidance for the assessment of exposure by inhalation of chemical agents for comparison with limit values and measurement strategy. European Standard EN 689, effective no later than Aug 1995 (English version) (Feb 1995). 20

21 Example A sample of six full-shift TWA welding fume measurements resulted in the following statistics: (sample) geometric mean is 1.06 mg/m 3 (sample) geometric standard deviation is 1.83 What is the point estimate (i.e., best estimate) of the true 95 th percentile? 21

22 90 th, 95 th, and 99 th Percentiles 22

23 95 th Percentile 23

24 Alternative upper percentile formula 24

25 Focus on Upper Tail 25

26 III. Upper Confidence Limit (UCL) for the Sample 95 th Percentile Calculate confidence intervals around estimates of upper percentile (normal & lognormal) Confidence intervals are used to express uncertainty test hypotheses: to determine our confidence level that the SEG is in compliance with an OEL to determine our confidence level that the true 95 th percentile exposure is within a specific exposure control category 26

27 For single shift, TWA exposure limits (TWA OELs) focus on the upper tail of the distribution e.g., 95 th percentile exposure 27

28 Upper Percentile (e.g., 95 th percentile) Concept Calculate the 95% upper confidence interval for the 95th percentile statistic (upper tolerance limit) Application 95%UCL can be used to test the following hypotheses: H o : 95th percentile > OEL H a : 95th percentile < OEL Interpretation t ti If the 95%UCL is less than the OEL, then we can say that we are at least 95% confident that the true 95th percentile is less than the OEL 28

29 95%UCL for the 95 th Percentile Procedure: Calculate the gm and gsd Using n, read the UCL K-value from the appropriate table γ = confidence level, e.g., 0.95 p = proportion, e.g., 0.95 n = sample size Using gg gm,,g gsd, and k, calculate the 95%UCL y = ln( gm ) s y = ln( gsd ) _ 29

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33 IV. Rule-of-thumb for Eyeballing Exposure Data Given: G = median X p = G x D Zp (e.g., X 0.95 =G x D ) a Rule-of-thumb, or guideline, can be devised d for quickly estimating from limited data the range in which the true 95 th percentile might lie. 33

34 Multiple of GM (median) GSD X p = 95 th percentile Z p =

35 R.O.T. for Estimating the 95 th Percentile 1. If n is small (i.e., <6) and one or more measurements > OEL, then decision i = Category Estimate the median and use it as a surrogate of the sample GM: - Sort the data - If n is odd the median is the middle value. - If n is even the median is the average of two middle values. 3. Multiply the median by 2, 4, and 6 - The results comprise an approximate low, middle, and high estimate of X

36 Rule-of-thumb Workshop (assume OEL=100) a. X = {5} b. X = {68} c. X = {7, 34, 57} d. X = {1, 1, 2, 5} e. X = {4, 5, 8, 23} f. X = {0.3, 1, 2, 3, 4, 22} g. X = {10, 10, 10, 20, 50, 105} h. X = {7, 10, 16, 21, 45, 53} For each dataset, determine the appropriate Exposure Category 1, 2, 3, or 4 using the above Rule-of-thumb. 36

37 Available Data Analysis Tools IHStats.xls Comes with the AIHA 3 rd Edition Exposure Assessment and Management handles n<50 EASC-IHStats.xls An update of the IHStats.xls spreadsheet handles n<200 multiple languages 37

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