Page:NIOSH Manual of Analytical Methods - 7400.pdf/10

 ASBESTOS and OTHER FIBERS by PCM: METHOD 7400, Issue 2, dated 15 August 1994 - Page 10 of 15

a group of laboratories, there is a 90% probability that the mean would fall between 0.01 and 0.08 fiber/ mL. These limits should be reported in any comparison of results between laboratories. Note that the of 0.45 used to derive Figure 1 is used as an estimate for a random group of laboratories. If several laboratories belonging to a quality assurance group can show that their interlaboratory is smaller, then it is more correct to use that smaller. However, the estimated of 0.45 is to be used in the absence of such information. Note also that it has been found that can be higher for certain types of samples, such as asbestos cement [9]. Quite often the estimated airborne concentration from an asbestos analysis is used to compare to a regulatory standard. For instance, if one is trying to show compliance with an 0.5 fiber/mL standard using a single sample on which 100 fibers have been counted, then Figure 1 indicates that the 0.5 fiber/mL standard must be 213% higher than the measured air concentration. This indicates that if one measures a fiber concentration of 0.16 fiber/mL (100 fibers counted), then the mean fiber count by a group of laboratories (of which the compliance laboratory might be one) has a 95% chance of being less than 0.5 fibers/mL; i.e., 0.16 + 2.13 × 0.16 = 0.5. It can be seen from Figure 1 that the Poisson component of the variability is not very important unless the number of fibers counted is small. Therefore, a further approximation is to simply use +213% and −49% as the upper and lower confidence values of the mean for a 100-fiber count. 90% Confidence Interval on Mean Count

Percent relative to single sample count

[subjective component (0.45) + Poisson component]

95% probability mean count is below this level

95% probability mean count is above this level

Number of fibers counted in a single sample Figure 1. Interlaboratory precision of fiber counts.

NIOSH Manual of Analytical Methods (NMAM), Fourth Edition