Should meta-analyses of interventions include observational studies in addition to randomized controlled trials? A critical examination of underlying principles.
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.
Shrier
Ian
I
Centre for Clinical Epidemiology and Community Studies, Lady Davis Institute for Medical Research, SMBD-Jewish General Hospital, McGill University, Montreal, Quebec, Canada. ian.shrier@mcgill.ca
Boivin
Jean-François
JF
Steele
Russell J
RJ
Platt
Robert W
RW
Furlan
Andrea
A
Kakuma
Ritsuko
R
Brophy
James
J
Rossignol
Michel
M
D015982:Bias; D015986:Confounding Factors, Epidemiologic; D036262:Empirical Research; D016021:Epidemiologic Studies; D006801:Humans; D015201:Meta-Analysis as Topic; D017410:Practice Guidelines as Topic; D016032:Randomized Controlled Trials as Topic