Issues in the design and conduct of clinical trials.

Careful consideration of issues in the design and conduct of clinical trials is essential for valid results to be achieved. Specifically, if the trial is randomized and well designed with respect to timing, pilot studies, choice of study population, sample size, duration of follow-up, and subject compliance, it can offer reliable evidence about a positive, inverse, or null effect of an intervention. With these considerations in mind, the Physicians' Health Study is being conducted among over 21,996 male physicians, 40-84 years of age, in the United States and involved a 2 X 2 factorial design used to test the roles of beta-carotene in the prevention of cancer and aspirin in the reduction of cardiovascular mortality. The choice of physicians as the study population and the implementation of a prerandomization run-in period resulted in 100% mortality and 99.5% morbidity follow-up, as well as 98.5% compliance with the treatment regimen 6 months after randomization. The ultimate goal of these methodologic considerations is to design studies that clearly can prove or refute the hypotheses that was tested.