Systematic reviews with language restrictions and no author contact have lower overall credibility: A methodology study

Background: High-quality systematic reviews (SRs) require rigorous approaches to identify, appraise, select, and synthesize research evidence relevant to a specific question. In this study, we evaluated the association between two steps in the conduct of an SR – restricting the search to English, and author contact for missing data – and the overall credibility of a SR. Methods: All SRs cited by the Endocrine Society's Clinical Practice Guidelines published from October 2006 through January 2012 were included. The main outcome was the overall A Measurement Tool to Assess Systematic Reviews (AMSTAR) score, as a surrogate of SR credibility. Nonparametric Kruskal–Wallis tests and multivariable linear regression models were used to investigate the association between language restriction, author contact for missing data, and the overall AMSTAR score. Results: In all, 69 SRs were included in the analysis. Only 31 SRs (45%) reported searching non-English literature, with an average AMSTAR score of 7.90 (standard deviation [SD] =1.64). SRs that reported language restriction received significantly lower AMSTAR scores (mean =5.25, SD =2.32) (P<0.001). Only 30 SRs (43%) reported contacting authors for missing data, and these received, on average, 2.59 more AMSTAR points (SD =1.95) than those who did not (P<0.001). In multivariable analyses, AMSTAR score was significantly correlated with language restriction (beta =-1.31, 95% confidence interval [CI]: -2.62, -0.01, P=0.05) and author contact for missing data (beta =2.16, 95% CI: 0.91, 3.41, P=0.001). However, after adjusting for compliance with reporting guidelines, language restriction was no longer significantly associated with the AMSTAR score. Conclusion: Fewer than half of the SRs conducted to support the clinical practice guidelines we examined reported contacting study authors or searched non–English literature. SRs that did not conduct these two steps had lower quality scores, suggesting the importance of these two steps for overall SR credibility.