To the Editor: In their meta-analysis of randomized controlled trials of folic acid supplementation and risk of cardiovascular disease, Dr Bazzano and colleagues1 provided a clear description of their inclusion criteria and study selection process. Using multiple databases when searching for studies that meet meta-analysis inclusion criteria may help avoid selection bias.2 Failure to search multiple health databases can result in missing up to half of the relevant literature.3
EMBASE and MEDLINE are the 2 most comprehensive databases used in meta-analyses. EMBASE focuses on drugs and pharmacology and contains more than 18 million records, including an index of more than 7000 journals from 70 countries. It may be a relevant source of negative studies, because European and non–English-language journals may be more likely to publish negative findings. However, Bazzano et al limited their literature search to MEDLINE. In addition, an important source for obtaining negative studies is hand searching of “grey literature” (such as abstracts from major meetings and symposiums, newsletters, and theses). Omitting gray literature from meta-analyses can affect analysis outcome by as much as 15%.4
Financial Disclosures: None reported.
Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature
Use interactive graphics and maps to view and sort country-specific infant and early dhildhood mortality and growth failure data and their association with maternal
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