What contributions do languages other than English make on the results of meta-analyses?

Including only a portion of all available evidence may introduce systematic errors into the meta-analytic process and threaten its validity. We set ou t to examine whether language restricted meta-analyses, compared to languag e inclusive meta-analyses, provide different estimates of the effectiveness of interventions evaluated in randomized trials. We identified and retriev ed all 79 meta-analyses from several disease areas in which explicit eligib ility criteria regarding trial selection were reported. General characteris tics and quality of reporting of the meta-analyses were assessed using a va lidated instrument. We explored the effects of language of publication of t he randomized trials on the quantitative results using logistic regression analyses. Language restricted meta-analyses, compared to language inclusive meta-analyses, did not differ with respect to the estimate of benefit of t he effectiveness of an intervention (ROR = 0.98; 95% CI: 0.81-1.17). These results were also robust after a series of sensitivity analyses. This study provides no evidence that language restricted meta-analyses lead to biased estimates of intervention effectiveness. We encourage others to replicate this study using different sampling frames, clinical topics and interventio ns. (C) 2000 Elsevier Science Tnc. All rights reserved.