Analyzing the power and error of Listeria monocytogenes growth challenge studies

Domestic and international food safety policy developments have spurred interest in the design and interpretation of experimental growth challenge studies to determine whether ready-to-eat (RTE) foods are able to support growth of Listeria monocytogenes. Existing challenge study protocols and those under development differ markedly in terms of experimental design and the acceptance criteria under which a RTE food is determined not to support L. monocytogenes growth. Consequently, the protocols differ substantially with respect to the probability of incorrectly determining that growth occurs and the statistical power to detect growth if it does occur. Applying a fixed acceptance criteria exceedance value (e.g., less than a 0.5 log(10) or 1 log(10) increase) to distinguish real growth from quantitative measurement uncertainty over different experimental designs and/or measurement uncertainty values implies highly inconsistent type I error (alpha) probabilities. None of the L. monocytogenes growth challenge study designs currently being considered are likely to provide an F-test with alpha=0.05 and power >or=0.8 to detect a 1 log(10) increase in mean concentration over the entire range of measurement uncertainty values for enumeration of L. monocytogenes reported in food samples in a validation study of ISO Method 11290-2.