ANALYSIS OF DATA FROM GROUP-RANDOMIZED TRIALS WITH REPEAT OBSERVATIONS ON THE SAME GROUPS

This study used Monte Carlo simulations to evaluate the performance of alternative models for the analysis of group-randomized trials having more than two time intervals for data collection. The major distincti on among the models tested was the sampling variance of the interventi on effect. In the mixed-model ANOVA, the sampling variance of the inte rvention effect is based on the variance among group x time-interval m eans. In the random coefficients model, the sampling variance of the i ntervention effect is based on the variance among the group-specific s lopes. These models are equivalent when the design includes only two t ime intervals, but not when there are more than two time intervals. Th e results indicate that the mixed-model ANOVA yields unbiased estimate s of sampling variation and nominal type I error rates when the group- specific time trends are homogenous. However, when the group-specific time trends are heterogeneous, the mixed-model ANOVA yields downwardly biased estimates of sampling variance and inflated type I error rates . In contrast, the random coefficients model yields unbiased estimates of sampling variance and the nominal type I error rate regardless of the pattern among the groups. We discuss implications for the analysis of group-randomized trials with more than two time intervals. (C) 199 8 John Wiley & Sons, Ltd.