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Assessing the presence of selection bias in meta-analyses of randomised trials using baseline heterogeneity


To assess the likelihood of biased trials contained in 12 recently published meta-analyses.


A review of component randomised controlled trials of systematic reviews with 12 meta analyses.

Data sources

12 recently published systematic reviews with 503 component randomised trials, which were published in the BMJ, Lancet, JAMA and The Annals of Internal Medicine up until May 2012.

Eligibility criteria for selecting studies

Systematic reviews were eligible for inclusion if they included only randomised controlled trials (RCTs). We obtained the full text for the component RCTs of the 12 systematic reviews. We excluded RCTs which were not available in English from our review and analysis.


Five of the 12 meta-analyses exhibited heterogeneity (I2 >0.30) in age differences, when there should have been none, with two having significant or substantial heterogeneity (I2 > 0.50). In three meta-analyses we found that the age of intervention group was statistically significantly higher than in the control group. Two meta-analysis had a distribution of p-values that were inconsistent with chance. Meta-regression explained some of the observed heterogeneity in two meta-analyses as a consequence of poor allocation concealment.


The majority of a sample of recent meta-analyses showed that there were signs of imbalance and or heterogeneity in ages between treatment groups, when there should have been none. Systematic reviewers might consider using the techniques described here to assess the validity of their findings.

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Open Access This article is published under license to BioMed Central Ltd. This is an Open Access article is distributed under the terms of the Creative Commons Attribution License ( ), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver ( ) applies to the data made available in this article, unless otherwise stated.

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Clark, L., Fairhurst, C., Hewitt, C. et al. Assessing the presence of selection bias in meta-analyses of randomised trials using baseline heterogeneity. Trials 14 (Suppl 1), O96 (2013).

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