How many Cochrane reviews are needed to cover existing evidence on the effects of healthcare interventions?

Clinicians are faced with the challenge of keeping abreast of the rapidly growing clinical literature. One method that can help clinicians meet this challenge is the provision of synthesised evidence by such groups as the Cochrane Collaboration. The Cochrane Library contains the full text of Cochrane reviews, which bring together information on controlled studies (mostly randomised controlled trials) in a standard format. The first 1000 reviews were prepared by early 2001, and there are now more than 1600 available.1 The task of synthesising the clinical literature is a huge undertaking, and in this editorial we will use the number of studies in a typical Cochrane review to provide an estimate of the number of Cochrane reviews that are needed to cover all trials currently in the Cochrane Central Register of Controlled Trials (CENTRAL).2–3 CENTRAL is the main source used by Cochrane reviewers to locate studies. It contains citations to more than 300 000 reports of studies, which may be eligible for inclusion in Cochrane reviews. CENTRAL is reference-based rather than study-based, and therefore multiple references may be included for a single trial. We examined 1000 reviews from the Cochrane Database of Systematic Reviews in Issue 1, 2001, of the Cochrane Library to determine the average number of studies, and references to the studies, included in a typical Cochrane review. Comparing this with an estimate for the number of relevant references in CENTRAL, it is possible to predict how many more Cochrane reviews are needed to cover the healthcare research already in CENTRAL. Of the 1000 reviews, 11 had been replaced by notices of withdrawal, leaving 989 complete reviews. The included studies in these reviews were hand counted by 1 author …