Sample sizes for group sequential cohort and case-control study designs.

This paper proposes the use of group sequential methods of calculate sample sizes for cohort and case-control study designs. The methods are based upon the theory of repeated significance tests, one of several sequential methods currently available. Group sequential methods permit repeated significance testing of relative risks obtained from periodically accumulated data while maintaining the required overall level of significance. Tables are presented for cohort and case-control studies in which the average sample size required for a group sequential design is compared to that of the conventionl fixed sample size plan for the usual constant relative risk situation. The tables show that group sequential designs are in general more efficient than fixed sample size plans for cohort and case-control studies. Computer simulations showed that group sequential methods can be employed when stratified data analyses are to be used and when the sample sizes of the two study groups are unequal.