Direction and impact of language bias in meta-analyses of controlled trials: empirical study

Excluding clinical trials reported in languages other than English from meta-analyses may introduce bias and reduce the precision of combined estimates of treatment effects. We examined the influence of trials published in languages other than English on combined estimates and conclusions of published meta-analyses.

We searched journals and the Cochrane Database of Systematic Reviews for meta-analyses of at least five trials with binary outcomes that were based on comprehensive literature searches without language restrictions. We compared estimates of treatment effects from trials published in languages other than English to those from trials published in English, and assessed the impact of restricting meta-analyses to trials published in English.