4,000 high quality reviews and counting: a vast wealth of knowledge in The Cochrane Library

In 2001, The Cochrane Library published the 1,000th full Cochrane Review.[1] This came six years after the first few dozen Cochrane reviews appeared in the new Cochrane Database of Systematic Reviews.[2] Now, with Issue 4 2009, the number of full Cochrane Reviews has exceeded 4,000. Not only does this represent a tremendous achievement for the many people, institutions and entities who have contributed to the work, it also represents a vast wealth of healthcare research and knowledge.

In 2001, the typical Cochrane Review included six or seven trials.[2] The figure seems similar today, as does the average number of participants in the studies assessed within the review: at about 1,000 per review.[3] Thus, the 4,000 full Cochrane reviews are likely to use data from tens of thousands of studies and millions of participants. Furthermore, if one assumes that each primary study cost an average of 100,000 euros, which is likely an underestimate, the total cost of the research included in Cochrane Reviews would be billions of euros. Therefore, as the existing reviews are updated and new ones are published for the first time, The Cochrane Collaboration should celebrate its success in making the findings from this research, and the enormous commitment by members of the public, researchers and funding agencies it represents, widely available and accessible.

Mike Clarke, UK Cochrane Centre
Gerd Antes, German Cochrane Centre
David Moher, Bias Methods Group

1. Mallet S, Clarke M. The ‘average’ Cochrane systematic review: How many trials? How many participants? International Journal of Technology Assessment in Health Care 2002; 18: 820-823.
2. Starr M, Chalmers I, Clarke M, Oxman A. The origins, evolution, and future of The Cochrane Database of Systematic Reviews. International Journal of Technology Assessment in Health Care 2009; supplement 1: 182-195.
3. Moher D, Tetzlaff J, Tricco AC, Sampson M, Altman DG. Epidemiology and reporting characteristics of systematic reviews. PLoS Medicine 2007; 4(3): e78.