Planning group sizes in clinical trials with a continuous outcome and repeated measures.

Simple formulae have been derived for the accurate computation of sample sizes when the two-sample t-test is used. Required sample sizes may be reduced by taking repeated measures, especially if correlations are about equal. In a trial with two parallel groups, repeated measures may be summarized by a pre-treatment mean and a post-treatment mean. In a cross-over trial measures may be repeated within each treatment period. In case the difference between two independent groups increases in time, individual slopes may be used if the correlation between two measures hardly depends on distance in time between these measures. However, it may be more efficient to use only the first and last measure in the statistical test if correlations strongly depend on time distance.