Background: Systematic reviews aim to help people make well-informed decisions about healthcare interventions. However, systematic reviews often conclude that there is a lack of high quality studies and that more research is needed. Lack of evidence from reviews, especially if no primary studies were identified, ideally ought to initiate new research projects.
Objectives: The aim of this study was to identify the number of reviews with no included studies (empty reviews) in the Cochrane Database of Systematic Reviews Issue 1, 2007. We also intended to identify empty reviews in earlier versions, and to describe their history.
Methods: We screened all reviews (n=3009) online in the Cochrane Database of Systematic Reviews Issue 1, 2007. We extracted information about number of studies included in all reviews across review groups. We also screened other (earlier) versions of published reviews that were available in Issue 1, 2007 and reported number of included studies in each version.
Results: 214 reviews (14%) in Issue 1, 2007 were empty reviews. The number of empty reviews within one review group ranged from none to 25 (12%). The mean number of empty reviews per review group was 4 (SD 6). In nine review groups there were no empty reviews. By tracking information from earlier versions we only identified 31 reviews that were empty. Ten of these reviews were still empty in Issue 1, 2007. Generally, other versions of published reviews were inadequately reported or linked. It was impossible to track a valid history of empty reviews and to assess what happened to them.
Conclusions: 14% of reviews in Cochrane Database of Systematic Reviews Issue 1, 2007 were reviews with no included studies (empty reviews). The number of empty reviews varied across review groups with a range from none up to 12%. Due to inadequate reporting and linking, the history of empty reviews was impossible to describe.