Calculation of Dispersion and z-scores for Microbiology Proficiency Testing Based on Results of a Ministry of Agriculture, Forestry and Fisheries Project

We analyzed data from microbiology proficiency tests obtained from research laboratories participating in a research project administered by the Ministry of Agriculture, Forestry and Fisheries of Japan. The data obtained from most of the participating laboratories was satisfactory. However, the results from some laboratories did not meet the minimum requirements, even laboratories that had reported satisfactory scores in previous years. The results also showed that, although the standard deviation calculated based on normalized inter-quartile ranges of coliform counts was close to 0.25, the standard deviation obtained for aerobic bacterial counts differed significantly from 0.25, which is the empirical sigma(p) used by the FEPAS proficiency testing scheme.