Clinical trials of medical treatments often compare two treated groups or a treated group with a separate but concurrent control group. We have examined a consecutive series of 47 such parallel studies reported in the Journal in 1978–1979, including 35 with random assignment to the treated or control group, to discover how this approach is actually used. A major strength of these studies as a group was the frequent use of randomized treatment assignment. Common problems included lack of sufficient detail about methods of randomization, failure to provide enough detail about patient sources, and insufficient use of multivariate statistical techniques and of statistical modeling. We emphasize the importance of avoiding bias by balancing prognostic factors when assigning patients to treatments, reducing bias by modeling the influence of prognostic factors on response, and increasing precision by modeling. We also advocate the careful consideration of the relevance of a treatment comparison within the study to the external world of clinical practice. (N Engl J Med 1983;309:1291–8).
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