The FDA guideline indicates that a stability study protocol must describe (1) how the stability study is to be designed and carried out, and (2) statistical methods to be used in analyzing the data. The design of a stability study is intended to establish an expiration dating period, which is based on testing a limited number of batches of a drug product, applicable to all future batches of the drug product manufactured under similar circumstances. Therefore, the study design should be

chosen so that it can reduce bias and identify and control any expected and/or unexpected sources of variations. The goal for selection of an appropriate stability design is to improve the accuracy and precision of the established shelf life. On the other hand, statistical methods used for analysis of the data collected should reflect the nature of the design to provide a valid statistical inference for the established shelf life. In this chapter we focus on designs for long-term stability studies. Statistical methods for the analysis of stability data are provided in Chapters 11 and 12.