Objective: To assemble a large dataset of language restricted and language inclusive systematic reviews, including both conventional medicinal (CM) and complementary and alternative medicine (CAM) interventions. To then assess the quality of these reports by considering and comparing different types of systematic reviews and their associated RCTs; CM and CAM interventions; the effect of language restrictions compared with language inclusions, and whether these results are influenced by other issues, including statistical heterogeneity and publication bias, in the systematic review process.

Data sources: MEDLINE, EMBASE, the Cochrane Database of Systematic Reviews and the Centralised Information Service for Complementary Medicine.

Review methods: Three types of systematic reviews were included: language restricted; language inclusive/English language (EL) reviews that searched RCTs in languages other than English (LOE) but did not find any and, hence, could not include any, in the quantitative data synthesis; and systematic reviews that searched for RCTs in LOE and included them in the quantitative data synthesis. Fisher's exact test was applied to compare the three different types of systematic reviews with respect to their reporting characteristics and the systematic review quality assessment tool. The odds ratio of LOE trials versus EL trials was computed for each review and this information was pooled across the reviews to examine the influence that language of publication and type of intervention (CM, CAM) have on the estimates of intervention effect. Several sensitivity analyses were performed.

Results: The LOE RCTs were predominantly in French and German. Language inclusive/LOE systematic reviews were of the highest quality compared with the other types of reviews. The CAM reviews were of higher quality compared with the CM reviews. There were only minor differences in the quality of reports of EL RCTs compared with the eight other languages considered. However, there are inconsistent differences in the quality of LOE reports depending on the intervention type. The results, and those reported previously, suggest that excluding reports of RCTs in LOE from the analytical part of a systematic review is reasonable. Because the present research and previous efforts have not included every type of CM RCT and the resulting possibility of the uncertainty as to when bias will be present by excluding LOE, it is always prudent to perform a comprehensive search for all evidence. This result only applies to reviews investigating the benefits of CM interventions. This does not imply that systematic reviewers should neglect reports in LOE. We recommend that systematic reviewers search for reports regardless of the language. There may be merit in including them in some aspects of the review process although this decision is likely to depend on several factors, including fiscal and other resources being available. Language restrictions significantly shift the estimates of an intervention's effectiveness when the intervention is CAM. Here, excluding trials reported in LOE, compared with their inclusion, resulted in a reduced intervention effect. The present results do not appear to be influenced by statistical heterogeneity and publication bias.

Conclusions: With the exception of CAM systematic reviews, the quality of recently published systematic reviews is less than optimal. Language inclusive/LOE systematic reviews appear to be a marker for a better quality systematic review. Language restrictions do not appear to bias the estimates of a conventional intervention's effectiveness. However, there is substantial bias in the results of a CAM systematic review if LOE reports are excluded from it.