Validity of the percent reduction in standard deviation outlier test for screening laboratory means from a collaborative study.

AOAC INTERNATIONAL recommends an outlier test that is based on the "percent reduction in standard deviation" (PRSD) for screening the highest or lowest and 2-highest or 2-lowest laboratory means from a collaborative study. The original critical values that were developed to assess the significance of the test statistics associated with the PRSD test were obtained by simulation. In this paper, we answer several questions relative to the validity of the simulated critical values and develop formulas, based on the Student's t-distribution, to compare the simulated and formula-based critical values. Assumptions and derivations of formulas are provided along with selected tabular critical values that are used to test hypotheses at various levels of significance.