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Published online by Cambridge University Press:  26 April 2012

Andra Morrison
Canadian Agency for Drugs and Technologies in Health
Julie Polisena
Canadian Agency for Drugs and Technologies in Health email:
Don Husereau
Private Universität für Gesundheitswissenschaften, Medizinische Informatik und Technik GmbH University of Ottawa
Kristen Moulton
Canadian Agency for Drugs and Technologies in Health
Michelle Clark
Canadian Agency for Drugs and Technologies in Health
Michelle Fiander
Cochrane Effective Practice and Organization of Care (EPOC) Group
Monika Mierzwinski-Urban
Canadian Agency for Drugs and Technologies in Health
Tammy Clifford
Canadian Agency for Drugs and Technologies in Health and University of Ottawa
Brian Hutton
Ottawa Hospital Research Institute
Danielle Rabb
Canadian Agency for Drugs and Technologies in Health


Objectives: The English language is generally perceived to be the universal language of science. However, the exclusive reliance on English-language studies may not represent all of the evidence. Excluding languages other than English (LOE) may introduce a language bias and lead to erroneous conclusions.

Study Design and Setting: We conducted a comprehensive literature search using bibliographic databases and grey literature sources. Studies were eligible for inclusion if they measured the effect of excluding randomized controlled trials (RCTs) reported in LOE from systematic review-based meta-analyses (SR/MA) for one or more outcomes.

Results: None of the included studies found major differences between summary treatment effects in English-language restricted meta-analyses and LOE-inclusive meta-analyses. Findings differed about the methodological and reporting quality of trials reported in LOE. The precision of pooled estimates improved with the inclusion of LOE trials.

Conclusions: Overall, we found no evidence of a systematic bias from the use of language restrictions in systematic review-based meta-analyses in conventional medicine. Further research is needed to determine the impact of language restriction on systematic reviews in particular fields of medicine.

Copyright © Cambridge University Press 2012

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