A systematic review is a literature review focused on a single question which tries to identify, appraise, select and synthesize all high quality research evidence relevant to that question. Systematic reviews are generally regarded as the highest level of medical evidence by evidence-based medicine professionals. An understanding of systematic reviews and how to implement them in practice is becoming mandatory for all professionals involved in the delivery of health care.
A systematic review is a summary of healthcare research that uses explicit methods to perform a thorough literature search and critical appraisal of individual studies to identify the valid and applicable evidence. It often, but not always, uses appropriate techniques (meta-analysis) to combine these valid studies, or at least uses grading of the levels of evidence depending on the methodology used.
While many systematic reviews are based on an explicit quantitative meta-analysis of available data, there are also qualitative reviews which nonetheless adhere to the standards for gathering, analyzing and reporting evidence.
The Campbell Collaboration produces systematic reviews for the mission of "The Campbell Collaboration (C2) helps people make well-informed decisions by preparing, maintaining and disseminating systematic reviews in education, crime and justice, and social welfare."
Many healthcare journals now publish systematic reviews, but the best-known source is the Cochrane Collaboration, a group of over 6,000 specialists in health care who systematically review randomised trials of the effects of treatments and, when appropriate, the results of other research. Cochrane reviews are published in the Cochrane Database of Systematic Reviews section of the Cochrane Library, which to date (February 2007) contains 2,893 complete reviews and 1,646 protocols.
The Cochrane Group provides a handbook for systematic reviewers of interventions], where they suggest that each systematic review should contain the following main sections:
- Methods of the review
- Conclusion and discussion
Steps in conducting a systematic review
There are seven steps for preparing and maintaining a systematic review:
- Formulating a problem
- Locating and selecting studies. Deluca has published a search strategy.
- Critical appraisal of studies
- Collecting data
- Analyzing and presenting results
- Interpreting results
- Improving and updating reviews
Strengths and weaknesses
While systematic reviews are regarded as the strongest form of medical evidence and systematic reviews are cited more often than narrative reviews, a review of 300 studies found that not all systematic reviews were equally reliable, and that their reporting could be improved by a universally agreed upon set of standards and guidelines.
The Oxman scale has been developed to assess the quality of systematic reviews. The scale contains ten items (see decription of items at http://www.annals.org/cgi/content/full/147/7/492/T2).
A further study by the same group found that of 100 guidelines reviewed, 4% required updating within a year, and 11% after 2 years; this figure was higher in rapidly-changing fields of medicine, especially cardiovascular medicine. 7% of systematic reviews needed updating at the time of publication. 
Practicing physicians tend to rate systematic reviews as "higher for relevance to practice than articles with original evidence and lower for useful new information".
The logo for the Cochrane Collaboration is the scattergram that summarizes the result of an early meta-analysis of corticosteroids to prevent to prevent fetal lung disease. This meta-analysis contributed to the realization that many patients were not receiving a treatment whose benefit had already been established.
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- Cochrane Collaboration