Should meta-analysts search Embase in addition to Medline?
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It is widely accepted that meta-analysts should search multiple databases. The selection of databases is ideally based on the potential contribution of each database to the project or on the potential for bias if a database is excluded, as supported by research evidence. We explore whether searching Embase yields additional trials that influence a meta-analysis. We identified meta-analyses that searched Medline and Embase. A random-effects weighted mean method was used to estimate the intervention effect in articles indexed only in Embase compared with those indexed elsewhere. On average, Embase-unique trials yielded significantly smaller estimates by 29% (ratio of odds ratio [ROR] 0.71, 95% confidence interval [CI] 0.56-0.90) but influenced the pooled estimate by an average of only 6% (ROR 0.94, 95% CI 0.88-0.99). Searching Medline but not Embase risks biasing a meta-analysis by finding studies that show larger estimates, but their prevalence seems low enough that the risk may be slight, provided the rest of the search is comprehensive.
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