Sample size determination for proving equivalence based on the ratio of two means for normally distributed data

Equivalence trials aim to demonstrate that two treatments do not differ by more than a prespecified clinically irrelevant amount. We consider the prob lem when equivalence is defined in terms of the ratio of population means a nd the original (untransformed) data are normally distributed. Application of the intersection-union principle to the test proposed by Sasabuchi resul ts in a two one-sided tests procedure of size alpha. We give the associated 100 (1 - 2 alpha) per cent confidence interval and derive the exact method s for calculation of power and sample sizes for the parallel group design a nd the two-period cross-over. We present tables and figures of required sam ple sizes and achieved power. Copyright (C) 1999 John Wiley & Sons, Ltd.