Browsing by Author "DeVries, Todd Alan"
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Item Estimating the antimicrobial log reduction: part 1. quantitative assays(1999) DeVries, Todd Alan; Hamilton, Martin A.In quantitative antimicrobial assays, the responses are counts of viable microbes in two treatment groups. One group is treated with a chemical germicide and the other group is control, treated with an inactive chemical. This is part 1 of a pair of papers that pertain to assays that estimate the log reduction (LR), in the density of viable microbes, attributable to the germicide treatment (part 2 is concerned with presence=absence responses). Such assays are used by producers, consumers, and regulatory agencies to assess the ef®cacy of liquid germicides. We de®ne and compare the two different mathematical formulations for LR that are commonly used in practice when there are replicate density measurements. One LR parameter is based on the mean of the log-transformed densities; the other is based on the logarithm of the mean of densities. We build a statistical model relating microbial count data to the LR parameters, derive maximum likelihood and method of moments estimators for each LR parameter, and compare the estimators according to both their asymptotic characteristics and the results of a simulation study utilizing realistic sample sizes. Standard error formulas for the estimators are derived, and they are evaluated via simulation studies. The results of this investigation lead us to recommend the method of moments estimator, regardless of which de®nition of LR is chosen.Item Estimating the antimicrobial log reduction: part 2. presence/absence assays(1999) DeVries, Todd Alan; Hamilton, Martin A.This is part 2 of a pair of papers on antimicrobial assays conducted to estimate the log reduction (LR), in the density of viable microbes, attributable to the germicide. Two alternative de®nitions of LR were de®ned in part 1, one based on the mean of the log-transformed densities; the other is based on the logarithm of the mean of densities. In this paper, we evaluate statistical methods for estimating LR from an antimicrobial assay in which the responses are presence=absence observations at each dilution in a series of dilutions. We provide a model for the presence=absence data, and, for each de®nition of LR, we derive the maximum likelihood estimator (mle). Using computer simulation methods, we compare the mle to several alternative estimators, including an estimator based on averaging the log-transformed most probable number (mpn) values. Standard error formulas for the estimators are also derived and evaluated using computer simulations. This investigation results in the following recommendations. If the parameter of interest is based on the mean of log-transformed densities, then the results favor use of the log-transformed mpn method. If, however, the parameter of interest is based on the logarithm of the mean of densities, then the results show that the mle should be used.Item Hard surface carrier test as a quantitative test of disinfection: a collaborative study(1995-07) Hamilton, Martin A.; DeVries, Todd AlanItem Quantitative analysis of a presence/absence microbiological assay: the hard surface carrier test of disinfectant efficacy(1996-09) Hamilton, Martin A.; DeVries, Todd AlanItem Statistical methods in microbial disinfection assays(Montana State University - Bozeman, College of Letters & Science, 1997) DeVries, Todd Alan