Sample-size calculation for a log-transformed outcome measure

The outcome measure of interest in clinical trials sometimes requires trans formation to the logarithmic scale for analysis. This paper examines sample -size calculation for both independent groups and matched-pairs trials for log-transformed outcomes. For both types of trial, we demonstrate how the c alculation can be formulated in terms of a relative treatment effect and a statement of relative variability, both specified on the original scale of measurement. Por a comparison of two independent groups, the relative treat ment effect is the ratio of group geometric means (or alternatively, group arithmetic means) and the coefficient of variation is used as a summary of relative variability. For a matched-pairs comparison, the appropriate relat ive treatment effect is the geometric mean of the within-pair ratios, and r elative variability can be specified as an upper bound on within-pair ratio s under a null hypothesis of the relative effect being equal to 1 (i.e., no difference). We discuss the clinical study that motivated this work and de monstrate the application of the sample-size calculation to this study. (C) Elsevier Science Inc. 1999.