A Model for Statistical Evaluation of Precision Parameters of Microbiological Methods: Application to Dry Rehydratable Film Methods and IDF Reference Methods for Enumeration of Total Aerobic Mesophilic Flora and Coliforms in Raw Milk

Abstract

Abstract A new statistical approach for collaborative study data of microbiological methods Is proposed. This Includes a confirmatory test to the Polsson distribution of the number of colonies. In addition, 2 new statistical parameters are used to express precision as a percent of the original unit: the geometric relative standard deviation (GRSD) and the critical relative difference between 2 measurements (RD95). This statistical approach was applied to an Interlaboratory study to assess and compare the precision of both dry rehydratable film (PetrlfUrn® SM and Petrifllm® VRB) methods and International Dairy Federation (IDF) reference methods [total aerobic mesophilic plate count (TAMPC) and violet red bile lactose agar (VRBL) methods] for estimation of total bacteria and collform, respectively, in raw milk. Each of the 14 laboratories In the study analyzed 40 laboratory samples (20 different materials In blind duplicates) for total bacteria and collform counts by both the Petrifllm and standard methods. Repeatability standard deviations (In log10 unit) of TAMPC, Petrifllm SM, VRBL, and Petrifllm VRB were 0.106, 0.089, 0.219, and 0.171, respectively; their reproducibility standard deviations were 0.170,0.167,0.348, and 0.199, respectively.