Should meta-analysts search Embase in addition to Medline?
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.
Sampson
Margaret
M
Chalmers Research Group, Children's Hospital of Eastern Ontario Research Institute, 401 Smyth Road, Ottawa, ON K1H 8L1, Canada. msampson@uottawa.ca
Barrowman
Nicholas J
NJ
Moher
David
D
Klassen
Terry P
TP
Pham
Ba'
B
Platt
Robert
R
St  John
Philip D
PD
Viola
Raymond
R
Raina
Parminder
P
D016206:Databases, Bibliographic; D006801:Humans; D016239:MEDLINE; D015201:Meta-Analysis as Topic; D016017:Odds Ratio; D010506:Periodicals as Topic; D016032:Randomized Controlled Trials as Topic; D015983:Selection Bias