ESTIMATING SAMPLE SIZES FOR REPEATED MEASUREMENT DESIGNS

Formulas for estimating sample sizes that are required to provide spec ified power for analysis of variance (ANOVA) tests of significance in a two-group repeated measurements design are presented and evaluated. Power and sample size requirements depend on the pattern of treatment effects and the pattern of correlations among the repeated measurement s, as well as on parameters common to sample size estimation for cross -sectional comparisons of treatment effects in simple randomized desig ns. Simplifying assumptions permit generation of these numerous parame ter estimates from predictions of the magnitude of the standardized '' effect size'' at end of trial and the single correlation between the b aseline and endpoint measurements. Monte Carlo methods are used to ver ify the actual power of different tests of significance for treatment effects in repeated measurement designs using sample sizes estimated b y the formulas. The sample size implications of different patterns of treatment effects, levels of correlation, and numbers of repeated meas urements are evaluated.