[Conservative experimental groups in clinical comparative trials. Tables providing the minimum number of patients]

To calculate the minimum size of sample required for clinical trials it is first necessary to decide the expected difference between the groups, and then the acceptable risk of wrongly declaring a significant result (alpha) or wrongly declaring a non-significant result (beta). Once these three parameters are set, simple tables can show the necessary sample sizes. Such tables are presented for classical comparative studies, survival studies, and group sequential plans. Sequential plans often entail large savings in sample size compared with fixed group plans.