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May 16, 1977


Author Affiliations

State University of New York Upstate Medical Center Syracuse

JAMA. 1977;237(20):2187-2188. doi:10.1001/jama.1977.03270470023007

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To the Editor.—  In his tabular summary of associations between HLA specificities and various diseases, Ritzmann (236:2305,1976) determines "relative risk factors" from the percentages with positive reactions to specific HLA antigens among cases and controls in retrospective studies. In the accompanying text, the terms "prevalence," "incidence," and "risk" are used almost interchangeably to describe such associations. While this usage is understandable, it reflects a fashionable confusion in the current medical literature.We believe that most readers will agree substantially with the following three definitions pertaining to a given disease. (1) Prevalence: proportion (percent) with the disease in a defined population; (2) incidence: proportion (percent) of a population that acquires the disease in a defined period of time; (3) risk (instantaneous): rate of disease onset, generally expressed as new cases per person per unit time.That clinically important distinctions among these terms are frequently ignored or blurred in the current medical