Methods and Applications of Statistics in Clinical Trials, Volume 1: Concepts, Principles, Trials, and Designs

Chapter 66

Primary Efficacy Endpoint

Dean A. Follmann

66.1 Defining the Primary Endpoint

The primary endpoint is the outcome by which the effectiveness of treatments in a clinical trial is evaluated. As such, it plays a key role in the overall design of the study and needs to be carefully specified to ensure that the results of the clinical trial will be accurate and accepted by the medical community. Studies with poorly chosen endpoints can lead to the wrong conclusions or produce controversial results. A carefully chosen endpoint can ensure no bias. But for a study result to be acceptable to the medical community, the endpoint needs also to be meaningful—of either demonstrated or accepted relevance for the population and interventions of the trial.

A primary efficacy endpoint needs to be specified before the start of the clinical trial. This is important for several reasons. A primary endpoint is required to determine a sample size that ensures a clinically meaningful difference in the primary endpoint between the two randomized groups will be detected with high probability, generally 0.80 to 0.90. Another important reason for a priori specification is that, if a primary endpoint is specified after unblinded data have been observed, there is no way to ensure that choice of endpoint has not been influenced by the knowledge of its effect on the comparison of arms. One might consciously or subconsciously be attracted to a primary endpoint that corresponds to a small P-value for ...