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American Journal of Epidemiology Advance Access originally published online on September 15, 2008
American Journal of Epidemiology 2008 168(9):1073-1081; doi:10.1093/aje/kwn217
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American Journal of Epidemiology © The Author 2008. Published by the Johns Hopkins Bloomberg School of Public Health. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org.

PRACTICE OF EPIDEMIOLOGY

What Do Case-Control Studies Estimate? Survey of Methods and Assumptions in Published Case-Control Research

Mirjam J. Knol, Jan P. Vandenbroucke, Pippa Scott and Matthias Egger

Correspondence to M. J. Knol, Julius Center for Health Sciences and Primary Care, University Medical Center Utrecht, P.O. Box 85500, 3508 GA Utrecht, the Netherlands (e-mail: m.j.knol{at}umcutrecht.nl).

Received for publication December 7, 2007. Accepted for publication June 18, 2008.

To evaluate strategies used to select cases and controls and how reported odds ratios are interpreted, the authors examined 150 case-control studies published in leading general medicine, epidemiology, and clinical specialist journals from 2001 to 2007. Most of the studies (125/150; 83%) were based on incident cases; among these, the source population was mostly dynamic (102/125; 82%). A minority (23/125; 18%) sampled from a fixed cohort. Among studies with incident cases, 105 (84%) could interpret the odds ratio as a rate ratio. Fifty-seven (46% of 125) required the source population to be stable for such interpretation, while the remaining 48 (38% of 125) did not need any assumptions because of matching on time or concurrent sampling. Another 17 (14% of 125) studies with incident cases could interpret the odds ratio as a risk ratio, with 16 of them requiring the rare disease assumption for this interpretation. The rare disease assumption was discussed in 4 studies but was not relevant to any of them. No investigators mentioned the need for a stable population. The authors conclude that in current case-control research, a stable exposure distribution is much more frequently needed to interpret odds ratios than the rare disease assumption. At present, investigators conducting case-control studies rarely discuss what their odds ratios estimate.

case-control studies; epidemiologic methods; odds ratio

Abbreviations: STROBE, Strengthening the Reporting of Observational Studies in Epidemiology


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