Volume 14, Issue 6 (November 1986)
Precision Testing of Standardized Microbiological Methods
Tests performed on identical materials in identical circumstances do not yield identical results. This is attributed to unavoidable random errors inherent in every test procedure. Repeatability and reproducibility have been found sufficient for describing the precision of a method. Examples of interlaboratory precision experiments in the field of food microbiology are presented, which also demonstrate the different types of reference material that can be used. With a fluid, microbiologically stabilized by freezing, repeatability of aerobic and Enterobacteriaceae plate counts (expressed in 10log-units) appeared to be 0.28; reproducibility was 0.37 for both plate counts. The repeatability of Staphylococcus aureus plate counts varied between 0.26 and 0.52, reproducibility between 0.31 and 0.84, depending on the method of enumeration and the testing procedure. It is concluded that microbiological methods should be evaluated by precision tests, carried out on an international scale.