title: The 2 x 2 factorial design: its application to a randomized trial of aspirin and carotene in U.S. physicians.
abstract: The 2 x 2 factorial design calls for randomizing each participant to treatment A or B to address one question and further assignment at random within each group to treatment C or D to examine a second issue, permitting the simultaneous test of two different hypotheses. This design can increase the efficiency of large-scale clinical trials. The Physicians' Health Study, a randomized trial of aspirin and beta-carotene among U.S. physicians, illustrates some features and potential problems in the design and analysis of a factorial trial. The most common concern, interaction between treatments, is generally an advantage rather than a limitation of this design. Although such interactions are relatively uncommon, this design provides a means to measure an effect which otherwise might not be apparent. If the interaction is sufficiently severe, however, then loss of power is possible.
mesh_terms: D001241:Aspirin / Q000627:therapeutic use*; D001699:Biometry; D002318:Cardiovascular Diseases / Q000401:mortality / Q000517:prevention & control*; D002338:Carotenoids / Q000627:therapeutic use*; D002986:Clinical Trials as Topic / Q000379:methods*; D006801:Humans; D009369:Neoplasms / Q000453:epidemiology / Q000517:prevention & control*; D012107:Research Design; D014481:United States
chemical_list: D002338:Carotenoids; D001241:Aspirin