Factorial designs allow two or more therapies to be evaluated simultaneously in one trial.
The efficacy of each treatment regarding one or more outcomes may be measured. The factorial design is the only method that allows evaluation of interactions between treatments. Many trials have used a factorial structure, including the Physician s Health Study , which evaluated the effect of aspirin in preventing myocardial infarction and the effect of beta carotene in preventing cancer. Other studies have included combinations of three or more treatments.