Some considerations for the planning of total-community prevention trials--when is sample size adequate?

Despite the large accumulated experience of statisticians with sample-size calculations for clinical trials, little information is available on extending this methodology to total community trials, in which the units of randomization are total communities and surveillance methods are used to assess event rates. As in clinical trials, the asymptotic formula for total sample size is used. However, the assumptions underlying the usual method of computing the expected T-year even rate for the experimental group, pe, are no longer valid in total community trials: all emigrants (dropouts) from large communities cannot practically be identified or followed. Immigrants, similar to dropouts in clinical trials in that they are exposed to the treatment for only a portion of the period but are followed to the end of the study, present an additional problem. This paper presents a method for the computation of pe in total community trials, taking into account in- and out-migration as well as the determinants usually considered in clinical trials. Sample computations are presented, and general problems of design, execution and data analysis of total community prevention trials are briefly discussed.