Stability analysis with discrete responses

We consider the estimation of shelf life of a drug product when the stability data are discrete. When there is no batch-to-batch variation, the proposed shelf life estimator is an approximate 95% lower confidence bound of the true shelf life. In the presence of batch-to-batch variation, the proposed shelf life estimator is an approximate 95% lower prediction bound of the shelf life of future batches. As a result, the proposed shelf life is applicable to all future batches of the same drug product. Testing for batch-to-batch variation based on discrete responses is also discussed.