title: The effect of English-language restriction on systematic review-based meta-analyses: a systematic review of empirical studies.
abstract: OBJECTIVE
  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.
  
  
  METHODS
  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.
authors.affiliation: Canadian Agency for Drugs and Technologies in Health, Ottawa, Ontario, Canada.
authors.forename: Andra
authors.initials: A
authors.lastname: Morrison
authors.forename: Julie
authors.initials: J
authors.lastname: Polisena
authors.forename: Don
authors.initials: D
authors.lastname: Husereau
authors.forename: Kristen
authors.initials: K
authors.lastname: Moulton
authors.forename: Michelle
authors.initials: M
authors.lastname: Clark
authors.forename: Michelle
authors.initials: M
authors.lastname: Fiander
authors.forename: Monika
authors.initials: M
authors.lastname: Mierzwinski-Urban
authors.forename: Tammy
authors.initials: T
authors.lastname: Clifford
authors.forename: Brian
authors.initials: B
authors.lastname: Hutton
authors.forename: Danielle
authors.initials: D
authors.lastname: Rabb
mesh_terms: D003625:Data Collection; D016206:Databases, Bibliographic; D016208:Databases, Factual; D006801:Humans; D033181:Information Dissemination / Q000379:methods*; D007802:Language; D015201:Meta-Analysis as Topic; D017594:Publication Bias; D011643:Publishing; D012196:Review Literature as Topic