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ASBESTOS and OTHER FIBERS by PCM:METHOD 7400,Issue 2,dated 15 August 1994-Page 9 of 15 At this time,there is no independent means for assessing the overall accuracy of this method.One measure of reliability is to estimate how well the count for a single sample agrees with the mean count from a large number of laboratories.The following discussion indicates how this estimation can be carried out based on measurements of the interlaboratory variability,as well as showing how the results of this method relate to the theoretically attainable counting precision and to measured intra-and interlaboratory Sr.(NOTE:The following discussion does not include bias estimates and should not be taken to indicate that lightly loaded samples are as accurate as properly loaded ones). Theoretically,the process of counting randomly(Poisson)distributed fibers on a filter surface will give an Sr that depends on the number,N,of fibers counted: Sr =1/N'12 Thus Sr is 0.1 for 100 fibers and 0.32 for 10 fibers counted.The actual Sr found in a number of studies is greater than these theoretical numbers[17,19-211. An additional component of variability comes primarily from subjective interlaboratory differences.In a study of ten counters in a continuing sample exchange program,Ogden [15]found this subjective component of intralaboratory Sr to be approximately 0.2 and estimated the overall Sr by the term: [N+(0.2xN)2]'/' N Ogden found that the 90%confidence interval of the individual intralaboratory counts in relation to the means were+2 Sr and-1.5 Sr.In this program,one sample out of ten was a quality control sample. For laboratories not engaged in an intensive quality assurance program,the subjective component of variability can be higher. In a study of field sample results in 46 laboratories,the Asbestos Information Association also found that the variability had both a constant component and one that depended on the fiber count[141. These results gave a subjective interlaboratory component of Sr(on the same basis as Ogden's)for field samples of ca.0.45.A similar value was obtained for 12 laboratories analyzing a set of 24 field samples [211.This value falls slightly above the range of Sr(0.25 to 0.42 for 1984-85)found for 80 reference laboratories in the NIOSH PAT program for laboratory-generated samples[17]. A number of factors influence Sr for a given laboratory,such as that laboratory's actual counting performance and the type of samples being analyzed.In the absence of other information,such as from an interlaboratory quality assurance program using field samples,the value for the subjective component of variability is chosen as 0.45.It is hoped that the laboratories will carry out the recommended interlaboratory quality assurance programs to improve their performance and thus reduce the Sr. The above relative standard deviations apply when the population mean has been determined.It is more useful,however,for laboratories to estimate the 90%confidence interval on the mean count from a single sample fiber count(Figure 1).These curves assume similar shapes of the count distribution for interlaboratory and intralaboratory results[161. For example,if a sample yields a count of 24 fibers,Figure 1 indicates that the mean interlaboratory count will fall within the range of 227%above and 52%below that value 90%of the time.We can apply these percentages directly to the air concentrations as well.If,for instance,this sample(24 fibers counted) represented a 500-L volume,then the measured concentration is 0.02 fibers/mL(assuming 100 fields counted,25-mm filter,0.00785 mm2 counting field area).If this same sample were counted by NIOSH Manual of Analytical Methods(NMAM),Fourth Edition