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We establish alert and action limits for environmental monitoring programs in cleanrooms in the pharmaceutical industry. Heavily skewed count data, which exhibited both overdispersion compared to the Poisson distribution and an excess of zeroes, needed to be modelled. A hierarchical Poisson-gamma model is proposed for this purpose and compared to other models proposed in the literature, demonstrating a clear improvement in terms of fit and interpretation.
Christensen et al. (Tue,) studied this question.