Biomedical research uses a wide range of designs applied to problems in laboratory, clinical, and population settings. Whatever the nature of the study, a few key features — such as the admission rule, the method of allocating subjects to treatments, and the use of controls — largely determine the strength of scientific inferences. We used these and other features to classify the 332 Original Articles published in the New England Journal of Medicine during 1978–1979. This classification directs attention to critical aspects of study design and performance and can help in the choice of suitable research approaches and protocols. It emphasizes the critical role of the investigators' intent in performing and analyzing a study, and it alerts readers to important aspects of interpretation. We recommend that authors always report enough detail about their work for readers to apply this or a similar classification. Omission of such detail may limit the interpretation of a research study because a study that cannot be classified has probably been incompletely reported. (N Engl J Med 1984; 311:1482–7.).
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