Background: Overviews of systematic reviews (overviews) compile data from multiple systematic reviews (SRs) and are a new method of evidence synthesis.
Objectives: To describe the methodological approaches in overviews of healthcare interventions.
Methods: We searched four databases from 2000 to May 2010. We included studies if they: stated a clear objective; examined an intervention; used explicit methods to identify SRs; collected and analysed outcome data from the SRs; and intended to include only SRs. Two researchers independently screened studies and applied eligibility criteria. One researcher extracted data with verification by a second. We conducted a descriptive analysis.
Results: The search yielded 1661 citations; 70 overviews were included. The number of overviews increased from 1 in 2000 to 10 in 2009. The interventions were pharmacological (n =17), non-pharmacological (n = 42), or both (n = 11). Inclusion criteria were clearly stated in most overviews (n = 63). The majority searched at least two databases (n = 47), reported the years and databases searched (n =48), and provided key words (n = 48). Eighteen overviews included Cochrane SRs only. Two reviewers independently screened and completed full text review in 27 overviews. Methods of data extraction were reported in 32. Quality of individual studies was extracted from the original SRs in 11 overviews. Quality assessment of the SRs was performed in 32 overviews; at least 13 different tools were used with the Oxman and Guyatt tool most frequent (n = 12). The quality of evidence was assessed in 11 overviews. Most overviews provided a narrative or descriptive analysis of the included SRs; a quantitative analysis was conducted in only two cases. One overview conducted indirect analyses. Publication bias was discussed in 19 overviews.
Conclusions: This study shows considerable variation in the methods used for overviews. There is a need for methodological rigour and consistency in overviews, as well as empirical evidence to support the methods employed.