A Monte Carlo study of alternative responses to intraclass correlation in community trials: Is it ever possible to avoid Cornfield’s penalties

Strategies to avoid the penalties of extra variation and reduced degrees offreedom in community trials were compared in Monte Carlo simulations Three conditions were found necessary to ensure nominal Type I and II error rates: (a) Condition variation must be assessed against assignment unit variation, (b) the critical value for the test statistic must be based on the assignment unit degrees offreedom, and (c) estimation of negative intraclass correlations must be allowed in the analysis. Using other test statistics and other degrees of freedom, and fixing negative intraclass correlations at zero often gave Type I and II error rates far from their nominal levels. Cornfield (1978) described two penalties that community trials face as a result of their allocation of intact social groups rather than individuals to study conditions. The major penalty is that the treatment effect must be assessed against the between-group variance. Unfortunately, the between-group vari-AUTHORS ’ NOTE The work reported here was supported by grants from the National Cance