Journal of AOAC International
Volume 88, Issue 3, May 2005, Pages 830-841  

Experimental evaluation of different precision criteria applicable to microbiological counting methods

Lombard, B.^a , Cornu, M.^a , Lahellec, C.^b , Feinberg, M.H.^c

^a AFSSA-LERQAP, 23 Avenue du General De Gaulle, 94 706 Maisons-Alfort, France
^b AFSSA Direction Générale, 17 Ave du General Leclerc, 94 700 Maisons-Alfort, France
^c Institut National de la Recherche Agronomique (INRA), 16 Rue Claude Bernard, 75005 Paris, France

Abstract

The dispersion of microbiological counting measurements, when repeating the analysis on the same material both within a laboratory (repeatability) and between laboratories (reproducibility) can be characterized by the organization of interlaboratory studies, where several sets of identical test materials are sent to several laboratories. Using the example of data generated by an interlaboratory study on enumeration of Listeria monocytogenes in foods by the standardized reference method (colony-count technique), 2 types of robust estimators of reproducibility standard deviations, based on the median, were examined, in comparison with the classical estimators, based on the mean. Experimental evaluation indicated that the 3 approaches gave consistent results for most of the combinations. The usual log₁₀ transformation of the enumeration results was also questioned before these calculations were conducted.