Objective: To examine trends in study design and other characteristics of original research published in JAMA, Lancet, and the New England Journal of Medicine (NEJM) between 1971 and 1991.
Design: A retrospective cross-sectional study of original clinical research published in JAMA, Lancet, and NEJM during 1971, 1981, and 1991.
Measurements: Four hundred forty-four articles were independently reviewed by at least two investigators and classified according to study design and other preselected study characteristics. Changes over time were analyzed by chi-square tests for categorical variables and analysis of variance for continuous variables.
Main results: Clinical results doubled, from 17% of all articles in 1971 to 35% in 1991 (p < 0.004), while case series decreased from 30% to 4% (p < 0.0001). Of 118 clinical trials, randomized controlled trials increased from 31% to 76% (p < 0.003) and nonrandomized controlled trials decreased from 42% to 8% (p < 0.002). Multicenter studies increased from 10% to 39% (p < 0.0001) and the prevalence of health services research increased from none in 1971 to 12% in 1991 (p < 0.001). The proportion of the studies explicitly excluding women from the subject population decreased from 11% in 1971 to 3% in 1991 (p < 0.03). In 1991 7% of the studies were composed entirely of men subjects, while only 0.7% of the studies were specific to men's health. Twelve percent of the studies in 1991 were specific to women's health. Between 1971 and 1991 there was no change in the prevalence of women first authors or studies addressing women's or minorities' health issues.
Conclusions: Several important changes in clinical research studies published in JAMA, Lancet, and NEJM have taken place between 1971 and 1991. Clinical trials have increased in frequency, largely replacing studies containing ten or fewer subjects. Health services research has increased in prevalence, reflecting growing interest in studies addressing the delivery of health care. Our data support the hypothesis that exclusion of women from clinical research studies is an important contributor to the paucity of data concerning women's health.