Baseline characteristics of participants in the Physicians' Health Study: a randomized trial of aspirin and beta-carotene in U.S. physicians.

The Physicians' Health Study is a randomized, double-blind, placebo-controlled trial of primary prevention designed to assess the effects of low-dose aspirin on cardiovascular disease and of beta-carotene on risks of cancer. A total of 22,071 U.S. male physicians 40 to 84 years of age were randomized to one of four treatment groups: active aspirin and active beta-carotene, active aspirin and beta-carotene placebo, aspirin placebo and active beta-carotene, or both placebos. Whereas the beta-carotene component of the trial is ongoing, the blinded aspirin component was terminated early primarily because of a statistically extreme benefit of aspirin on first myocardial infarction. We obtained data relating to a large number of variables, including demographics, personal medical history, family history, health habits, and diet before randomization and compared them among the four treatment groups. As expected in a randomized trial of this sample size, the distribution of baseline characteristics was virtually identical among the treatment groups. This comparison indicates certainly no confounding by the baseline variables that were collected and suggests that other unmeasured or unknown potential confounders are also likely to be distributed evenly between the treatment groups. Thus, any observed differences in outcome between these groups likely result from the effects of the treatments themselves.