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Environmental Monitoring Based on a Hierarchical Poisson-Gamma Model

Summary: [This abstract is based on the authors' abstract.]

Cleanrooms, in the context of the pharmaceutical industry, are maintained for production and quality control, with environmental monitoring of the rooms to ensure and document the level of cleanliness. A hierarchical Poisson-gamma model is proposed for modeling heavily skewed count data, which exhibited both overdispersion compared to the Poisson distribution as well as an excess of zeroes. The proposed model is compared to other models that have been proposed in literature, demonstrating a clear improvement in terms of fit and interpretation. It is advised to test whether data show evidence of overdispersion as a prerequisite to modeling the variation. An appendix looks at the estimation of parameters and the likelihood-ratio test statistic.

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  • Topics:
  • Keywords: Monitoring,Hierarchical systems,Poisson distribution,Modeling,Likelihood methods,Testing,Estimation,Comparison,Clean rooms,Pharmaceutical industry
  • Author: Christensen, Antje; Melgaard, Henrik; Iwersen, Jorgen; Thyregod, Poul
  • Journal: Journal of Quality Technology