I have worked with Dr. James J. Chen at the National Center for Toxicological Research on estimation and testing of the shelf life of drugs in stability analyses with two factors. In stability analysis, the current FDA recommended procedure for estimating the shelf life (expiration dating period) of a drug is limited to a single packaging or strength of a product. However, since most of drug products have more than one strength and are marketed in more than one package, stability analysis must be carried out for every combination of design factors in every storage condition. We have developed a multi-way analysis of covariance procedure for the analysis of stability data from a multiple packages and/or strengths study.