Systematic reviews are like scientific investigations in themselves, using pre-planned methods and an assembly of original studies that meet their criteria as 'subjects'. They synthesize the results of an assembly of primary investigations using strategies that limit bias and random error.

One of the first steps in this type of review is a comprehensive search of all potentially relevant articles with a pre-defined search strategy. A quantitative systematic review (like the type found in Cochrane) often summarizes results using a statistical technique called meta-analysis when studies are comparable. This technique allows researchers to combine the results of several studies into a single estimate of their combined result. It is a key element of many systematic reviews or overviews. It is also what is meant when people talk about 'pooling data'.

Quantitative systematic reviews that use meta-analysis and search for all relevant articles differ greatly from narrative reviews, which are summaries of research that lack explicit descriptions of systematic methods. The evidence in narrative reviews can be tenuous, incomplete, or based on a biased citation of studies (Antman 1992). In addition, the quantitative techniques incorporated in systematic reviews are more likely to detect small, but clinically meaningful treatment effects (Cooper 1980).

Learn more about the Principles of Meta-analysis:
View a PowerPoint presentation on 'Meta-analysis for beginners ', written by CMSG editor and biostatistician George Wells. Or click here for PowerPoint slides from the Cochrane Collaboration Training Workshops on the Conduct of Meta-analyses Using Individual Patient Data

Cochrane reviews are acknowledged as the best single source of evidence about the effects of healthcare interventions. Rigorous methods are employed that provide a powerful tool for deriving meaningful conclusions from the data and help prevent errors in interpretation.

Conclusions from Cochrane reviews can be invaluable to many groups: health policy makers; health professionals who are preparing clinical practice guidelines; clinical practitioners trying to keep up to date with advances in health care knowledge; members of the public who wish to be informed about the evidence concerning health care practices; and health care researchers who need to define what is already known before starting new research.

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