The Cochrane Library

The Cochrane Library is a collection of databases containing high-quality, independent evidence pertinent to healthcare decision-making. A major component of the Cochrane Library is the Cochrane reviews, these represent the highest level of evidence available for guiding decisions on clinical treatment. In addition to Cochrane reviews, the Library provides other sources of information, including abstracts of other systematic reviews, technology assessments, economic evaluations, and publications on individual clinical trials – all the current evidence at a single site.

What is a systematic review?

To help identify which treatments are effective, which are not, and which are harmful, results from randomized trials need to be assessed in a careful and scientifically rigorous manner. Trials are first assessed for quality and those that meet pre-determined quality criteria are combined to produce a statistically more reliable result than individual trials, and one that is likely to be widely applicable. The methodology used to combine the results of different trials must be scientifically rigorous. A systematic review uses an explicit methodology previously defined in the protocol. This methodology include steps to minimize bias at all stages of the review process: identification of relevant studies, selection for inclusion in the analysis, and the collection and combination of study data. 

A systematic review does not always present a statistical analysis of the results from the included studies. A statistical analysis may be impossible if the designs of the studies are too different for their results to combined meaningfully or if the outcomes measured are not sufficiently similar. If the results of the individual studies can be combined statistically to produce an overall picture, the process and its outcome is called a meta–analysis. A meta–analysis can also be performed without a systematic review, simply by combining results of more than one trial. However, although such a meta–analysis will have greater mathematical precision than an analysis of any one of the component trials, it will be subject to biases arising from the study selection process, and may produce a mathematically precise, but clinically misleading, result.



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