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American Journal of Epidemiology Advance Access originally published online on August 21, 2007
American Journal of Epidemiology 2007 166(10):1203-1209; doi:10.1093/aje/kwm189
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American Journal of Epidemiology © The Author 2007. 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

Should Meta-Analyses of Interventions Include Observational Studies in Addition to Randomized Controlled Trials? A Critical Examination of Underlying Principles

Ian Shrier1, Jean-François Boivin1,2, Russell J. Steele1,3, Robert W. Platt2,4, Andrea Furlan5, Ritsuko Kakuma2, James Brophy2,6 and Michel Rossignol1,2

1 Centre for Clinical Epidemiology and Community Studies, Lady Davis Institute for Medical Research, SMBD-Jewish General Hospital, McGill University, Montreal, Quebec, Canada
2 Department of Epidemiology and Biostatistics, McGill University, Montreal, Quebec, Canada
3 Department of Mathematics and Statistics, McGill University, Montreal, Quebec, Canada
4 Department of Pediatrics, McGill University, Montreal, Quebec, Canada
5 Institute for Work and Health, Toronto, Ontario, Canada
6 Department of Medicine, McGill University Health Centre, McGill University, Montreal, Quebec, Canada

Correspondence to Dr. Ian Shrier, Centre for Clinical Epidemiology and Community Studies, SMBD-Jewish General Hospital, 3755 Cote Ste Catherine Road, Montreal, Quebec H3T 1E2, Canada (e-mail: ian.shrier{at}mcgill.ca).

Received for publication December 13, 2006. Accepted for publication May 25, 2007.

Some authors argue that systematic reviews and meta-analyses of intervention studies should include only randomized controlled trials because the randomized controlled trial is a more valid study design for causal inference compared with the observational study design. However, a review of the principal elements underlying this claim (randomization removes the chance of confounding, and the double-blind process minimizes biases caused by the placebo effect) suggests that both classes of study designs have strengths and weaknesses, and including information from observational studies may improve the inference based on only randomized controlled trials. Furthermore, a review of empirical studies suggests that meta-analyses based on observational studies generally produce estimates of effect similar to those from meta-analyses based on randomized controlled trials. The authors found that the advantages of including both observational studies and randomized studies in a meta-analysis could outweigh the disadvantages in many situations and that observational studies should not be excluded a priori.

intervention studies; meta-analysis; observation; randomized controlled trials

Abbreviations: RCT, randomized controlled trial


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