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To examine whether the association of inadequate or unclear allocation concealment and lack of blinding with biased estimates of intervention effects varies with the nature of the intervention or outcome.Combined analysis of data from three meta-epidemiological studies based on collections of meta-analyses.146 meta-analyses including 1346 trials examining a wide range of interventions and outcomes.Ratios of odds ratios quantifying the degree of bias associated with inadequate or unclear allocation concealment, and lack of blinding, for trials with different types of intervention and outcome. A ratio of odds ratios <1 implies that inadequately concealed or non-blinded trials exaggerate intervention effect estimates.In trials with subjective outcomes effect estimates were exaggerated when there was inadequate or unclear allocation concealment (ratio of odds ratios 0.69 (95% CI 0.59 to 0.82)) or lack of blinding (0.75 (0.61 to 0.93)). In contrast, there was little evidence of bias in trials with objective outcomes: ratios of odds ratios 0.91 (0.80 to 1.03) for inadequate or unclear allocation concealment and 1.01 (0.92 to 1.10) for lack of blinding. There was little evidence for a difference between trials of drug and non-drug interventions. Except for trials with all cause mortality as the outcome, the magnitude of bias varied between meta-analyses.The average bias associated with defects in the conduct of randomised trials varies with the type of outcome. Systematic reviewers should routinely assess the risk of bias in the results of trials, and should report meta-analyses restricted to trials at low risk of bias either as the primary analysis or in conjunction with less restrictive analyses.

Original publication

DOI

10.1136/bmj.39465.451748.ad

Type

Journal article

Journal

BMJ (Clinical research ed.)

Publication Date

03/03/2008

Volume

336

Pages

601 - 605

Addresses

Department of Social Medicine, University of Bristol, Bristol BS8 2PR.

Keywords

Odds Ratio, Double-Blind Method, Bias (Epidemiology), Meta-Analysis as Topic, Randomized Controlled Trials as Topic