Comparison of two robust statistic methods used in inspection data of proficiency testing and evaluation of its results rationality
LI Yu-wu1 , SUN Pei-qin2, SUN Hai-rong 2
1. National Research Center for Environmental Analysis and Measurements, Beijing 100029, China; 2. National Accreditation Service for Conformity Assessment, Beijing 100062, China
Abstract：Quartile and iteration are two robust statistic methods used to calculate target standard deviation of data of proficiency testing, whose results are directly applied to evaluate whether the laboratory data are qualified.The analysis results of the two robust statistic methods are compared based on 195 sets of data (more than 100 test items) of proficiency testing.The calculation results of both simulation and real data show that the results of two methods are basically consistent when data tend to normal distribution.But when the testing item data obviously deviates from normal distribution, the standard deviation from quartile method is so strict that the pass rate of laboratory data is obviously decreased and it appears to make statistically "abandoning true" mistake.The rationality of two methods is evaluated based on the results from classic statistics deleted outliers.The average of relative deviation between two results results is 5-7% , which means the two results are relatively closed to each other for solid samples.The relative deviation of two methods’ results is 13.8% for liquid samples, which means the difference of two methods is relatively large.Iteration method not only reduces the effect of outliers on statistical analysis results but also avoids the human factor in the identification of outliers.It is suggested that the iteration method should be applied in domestic standards system in proficiency testing as soon as possible.
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