Selecting the appropriate statistical distribution for the primary analysis: a case study

An article in The Lancet discussed a clinical trial of a product for a rare disease. The authors had modified the primary analysis from an unadjusted Wilcoxon rank sum test to a Poisson regression. This led to several questions including: Was a Poisson distribution appropriate? How were the covariates selected? What is the effect of outliers (there were some)? If the Poisson model is not appropriate, can a permutation test approach provide information regarding the effect of the treatment? The presentation will show how Stata was used to evaluate these assumptions and provide an alternative analysis. The implications of this analysis are discussed.