A semi-empirical precision control criterion for duplicate microbial colony counts

A simple precision criterion (Z) of the logarithmic range (R) of duplicate microbiological determinations was developed. It is based on a freely selected relative technical uncertainty and the observed mean colony count. The model takes into account the different character of the technical and distribution uncertainty. The ensuing non-linear control criterion avoids the unrealistic assumptions of constancy that the pure Poisson and 'black box' (APHA) models make. The standardized variable Q = R/sigma R provides a linear control chart limit that enables sequential (temporal) plotting of the data.