Optimizing the literature search: coverage of included references in systematic reviews in Medline and Embase.

Objective: The aim of this study was to investigate if the included references in a set of completed systematic reviews are indexed in Ovid MEDLINE and Ovid Embase, and how many references would be missed if we were to constrict our literature searches to one of these sources, or the two databases in combination.

Methods: We conducted a cross-sectional study where we searched for each included reference (n = 4,709) in 274 reviews produced by the Norwegian Institute of Public Health to find out if the references were indexed in the respective databases. The data was recorded in an Excel spreadsheet where we calculated the indexing rate. The reviews were sorted into eight categories to see if the indexing rate differs from subject to subject.

Results: The indexing rate in MEDLINE (86.6%) was slightly lower than in Embase (88.2%). Without the MEDLINE records in Embase, the indexing rate in Embase was 71.8%. The highest indexing rate was achieved by combining both databases (90.2%). The indexing rate was highest in the category "Physical health - treatment" (97.4%). The category "Welfare" had the lowest indexing rate (58.9%).

Conclusion: Our data reveals that 9.8% of the references are not indexed in either database. Furthermore, in 5% of the reviews, the indexing rate was 50% or lower.