In the limitations sections of systematic reviews on any topic, it is not uncommon for the authors to discuss how language limitations within their search may have restricted the breadth of evidence presented. For instance, if the reviewers speak only English, the review is likely limited to publications and journals in that language. But how much of a difference does such a limitation make in terms of the overall conclusions of a systematic review? According to a new paper in the Journal of Clinical Epidemiology, probably not much - but it may depend on the specific topic of medicine under investigation.
While other methods reviews have previously examined this question, Dobrescu and colleagues extended the range of topics to methods reviews that included systematic reviews within the realm of complementary and alternative medicine, yielding four reviews previously unexamined by prior studies. Specifically, the authors looked for methods reviews comparing the restriction of literature searches to English-only versus unrestricted searches and whose primary outcomes compared differences in treatment effect estimates, certainty of evidence ratings, or conclusions based on the language restrictions enforced.
The search yielded eight studies investigating the impact of language restrictions in anywhere from 9 to 147 systematic reviews in medicine. Overall, the exclusion of non-English articles had a greater impact on estimates of treatment effects and the statistical significance of findings in reviews of complementary and alternative medicine versus conventional medicine topics. Most commonly, the exclusion of non-English studies led to a loss of statistical significance in these topic areas.
Overall, the methods studies examined found that the exclusion of non-English studies of conventional medicine topics led to small to moderate changes in the estimate of effect; however, exclusion of non-English studies shrank the observed effect size in complementary and alternative medicine topics by 63 percent. Two studies examined whether language restricted influenced authors' overall conclusions, generally finding no effect.
The authors conclude that when it comes to systematic reviews of conventional medicine topics, their findings are in line with those of previous methods studies which demonstrate little to no effect of language restrictions and suggest that restricting a search to English-only should not greatly impact the findings or conclusions of a review. However, the effect appears greater in the realm of complementary and alternative medicine, perhaps due to the greater proportion of non-English studies published in this field. Thus, systematic reviewers attempting to synthesize the evidence on an alternative medicine topic should be cognizant of their choices regarding language restriction and the potential implications they may have on their ultimate findings.
Dobrescu A, Nussbaumer SB, Klerings I et al. (2021). Restricting evidence syntheses of interventions to English-language publications is a viable methodological shortcut for most medical topics: A systematic review: Excluding English-language publications a valid shortcut. J Clin Epidemiol, epub ahead of print.
Manuscript available from publisher's website here.