Randomized Controlled Trials 6: Determining the Sample Size and Power for Clinical Trials and Cohort Studies

Performing well-powered, randomized, controlled trials is of fundamental importance in clinical research. The goal of sample size calculations is to assure that statistical power is sufficiently high when the probability of falsely rejecting a true null hypothesis (type I error) is kept acceptably small. This chapter overviews the fundamental of sample size calculation for standard types of outcomes for 2-group studies. It also considers (1) the problem of determining the size of the treatment effect that a study should be designed to detect, (2) modifications to sample size calculations to account for loss to follow-up and nonadherence, (3) options that can be used when initial calculations indicate that the feasible sample size is insufficient to provide adequate power, (4) implications of using multiple primary end points. In addition, a discussion of cluster randomized trials is provided. Sample size estimates for longitudinal cohort studies must take account of confounding by baseline factors.