The bias and accuracy factors introduced by Ross [Ross, T., 1996. Indices for performance evaluation of predictive models in food microbiology. J. Appl Bacteriol. 81, 501-508] for the evaluation of the performance of models in 'predictive food microbiology' are refined by basing the calculation of those measures on the mean square differences between predictions and observations. The use of the indices is extended by presenting formulae and methods which enable evaluation of the difference between alternative models for growth of an organism of interest over a domain of environmental factors. This is done by calculating the integral mean of the square differences between the models under investigation over the domain of the environmental variables common to those models, or a sub-region of it. The use of the techniques is exemplified by evaluating the difference between four published models for the growth rate of psychrotrophic pseudomonads.