EXTENSION OF DUNNETTS MULTIPLE COMPARISON PROCEDURE WITH DIFFERING SAMPLE SIZES TO THE CASE OF SEVERAL GROUPS

Dunnett's widely-used (Journ. Amer. Statist. Assoc. 50, 1955) procedure for one and two-sided comparisons of all active treatments with a control while maintaining a designated overall Type I error rate alpha was extended by Cheung and Holland (Biometrics 47, 1991) to the situation where one wishes to make all such comparisons simultaneously in each of r groups while maintaining alpha control over the probability of making any Type I errors. Whereas Cheung and Holland (1991) assumed a common sample size for all treatments in every group, the present paper discusses the frequently-encountered case where the treatment sample sizes may differ. The probability distributions of the appropriate statistics are derived for this case, and tables of the upper percentage points of these distributions are constructed with the aid of Dunnett's (Applied Statistics 38, 1989) algorithm. The application of this new procedure is illustrated with the reanalysis of sample data from an animal physiology experiment.