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Previous Volume 342:1887-1892 June 22, 2000 Number 25 Next

	Full Text PDF Editorial by Pocock, S. J. Letters Add to Personal Archive Add to Citation Manager Notify a Friend E-mail When Cited PubMed Citation

ABSTRACT

Background In the hierarchy of research designs, the results of randomized, controlled trials are considered to be evidence of the highest grade, whereas observational studies are viewed as having less validity because they reportedly overestimate treatment effects. We used published meta-analyses to identify randomized clinical trials and observational studies that examined the same clinical topics. We then compared the results of the original reports according to the type of research design.

Methods A search of the Medline data base for articles published in five major medical journals from 1991 to 1995 identified meta-analyses of randomized, controlled trials and meta-analyses of either cohort or case–control studies that assessed the same intervention. For each of five topics, summary estimates and 95 percent confidence intervals were calculated on the basis of data from the individual randomized, controlled trials and the individual observational studies.

Results For the five clinical topics and 99 reports evaluated, the average results of the observational studies were remarkably similar to those of the randomized, controlled trials. For example, analysis of 13 randomized, controlled trials of the effectiveness of bacille Calmette–Guérin vaccine in preventing active tuberculosis yielded a relative risk of 0.49 (95 percent confidence interval, 0.34 to 0.70) among vaccinated patients, as compared with an odds ratio of 0.50 (95 percent confidence interval, 0.39 to 0.65) from 10 case–control studies. In addition, the range of the point estimates for the effect of vaccination was wider for the randomized, controlled trials (0.20 to 1.56) than for the observational studies (0.17 to 0.84).

Conclusions The results of well-designed observational studies (with either a cohort or a case–control design) do not systematically overestimate the magnitude of the effects of treatment as compared with those in randomized, controlled trials on the same topic.

Source Information

From the Departments of Internal Medicine (J.C., N.S., R.I.H.) and Epidemiology and Public Health (R.I.H.), Yale University School of Medicine, New Haven, Conn.; and the Clinical Epidemiology Unit, West Haven Veterans Affairs Medical Center, West Haven, Conn. (J.C.).

Address reprint requests to Dr. Concato at the Department of Internal Medicine, Yale University School of Medicine, 333 Cedar St., SHM IE-61, New Haven, CT 06510, or at john.concato{at}yale.edu.

Full Text of this Article

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Observational Studies and Randomized Trials
Kunz R., Khan K. S., Neumayer H.-H., Sacks H. S., Liu P.-Y., Anderson G., Crowley J. J., Friedman H. S., Smith R. P., Meier P., Benson K., Hartz A. J., Concato J., Shah N., Horwitz R. I.
Extract | Full Text  
N Engl J Med 2000; 343:1194-1197, Oct 19, 2000. Correspondence

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