IJE Advance Access originally published online on May 21, 2007
International Journal of Epidemiology 2007 36(4):847-857; doi:10.1093/ije/dym087
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Impact of allocation concealment on conclusions drawn from meta-analyses of randomized trials
1The Nordic Cochrane Centre, Rigshospitalet, DK.
2Department of Biostatistics, University of Copenhagen, DK.
3Centre for Statistics in Medicine, Oxford, UK.
*Corresponding author. The Nordic Cochrane Centre, Rigshopitalet, 3343 Blegdamsvej 9, DK-2100 Copenhagen, Denmark. E-mail: jp{at}cochrance.dk
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Abstract |
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Background Randomized trials without reported adequate allocation concealment have been shown to overestimate the benefit of experimental interventions. We investigated the robustness of conclusions drawn from meta-analyses to exclusion of such trials.
Material Random sample of 38 reviews from The Cochrane Library 2003, issue 2 and 32 other reviews from PubMed accessed in 2002. Eligible reviews presented a binary effect estimate from a meta-analysis of randomized controlled trials as the first statistically significant result that supported a conclusion in favour of one of the interventions.
Methods We assessed the methods sections of the trials in each included meta-analysis for adequacy of allocation concealment. We replicated each meta-analysis using the authors' methods but included only trials that had adequate allocation concealment. Conclusions were defined as not supported if our result was not statistically significant.
Results Thirty-four of the 70 meta-analyses contained a mixture of trials with unclear or inadequate concealment as well as trials with adequate allocation concealment. Four meta-analyses only contained trials with adequate concealment, and 32, only trials with unclear or inadequate concealment. When only trials with adequate concealment were included, 48 of 70 conclusions (69%; 95% confidence interval: 56–79%) lost support. The loss of support mainly reflected loss of power (the total number of patients was reduced by 49%) but also a shift in the point estimate towards a less beneficial effect.
Conclusion Two-thirds of conclusions in favour of one of the interventions were no longer supported if only trials with adequate allocation concealment were included.
Keywords Bias (epidemiology), double-blind method, methods, randomized controlled trials, meta-analysis
Accepted 27 March 2007
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