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A systematic review is a literature review focused on a single question that tries to identify, appraise, select and synthesize all high quality research evidence relevant to that question. Systematic reviews of high-quality randomized controlled trials are crucial to evidence-based medicine. 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. Systematic reviews are not limited to medicine and are quite common in other sciences such as psychology, educational research and sociology.
A systematic review is a summary of 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 is often applied in the biomedical or healthcare context, but systematic reviews can be applied in any field of research and groups like the Campbell Collaboration are promoting their use in policy-making beyond just healthcare. It often, but not always, uses statistical techniques (meta-analysis) to combine these valid studies, or at least uses grading of the levels of evidence depending on the methodology used.
A systematic review uses an objective and transparent approach for research synthesis, with the aim of minimising bias. While many systematic reviews are based on an explicit quantitative meta-analysis of available data, there are also qualitative reviews which adhere to the standards for gathering, analyzing and reporting evidence. The EPPI-Centre have been influential in developing methods for combining both qualitative and quantitative research in systematic reviews.
Recent developments in systematic reviews include realist reviews, developed by Ray Pawson and Trisha Greenhalgh, and the meta-narrative approach by Greenhalgh and colleagues. These approaches try to overcome the problems of methodological and epistemological heterogeneity in the diverse literatures existing on some subjects.
Many healthcare journals now publish systematic reviews, but the best-known source is The Cochrane Collaboration, a group of over 15,000 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 (January 2009) contains 3,625 complete reviews and 1,921 protocols for additional reviews being conducted.
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
There are seven steps for preparing and maintaining a systematic review, as outlined in the Cochrane Handbook:
- Formulating a problem
- Locating and selecting studies
- 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, 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.
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.  A 2003 study suggested that extending searches beyond major databases, perhaps into gray literature, would increase the effectiveness of reviews.
- What is EBM. Centre for Evidence Based Medicine. URL accessed on 2009-03-18.
- Thomas J, Harden A, Oakley A, Oliver S, Sutcliffe K, Rees R. Brunton G. Kavanagh J (2004) Integrating qualitative research with trials in systematic reviews: an example from public health. British Medical Journal 328:1010-1012
- Fraser MacFarlane; Olivia Kyriakidou; Bate, Paul; Richard Peacock; Greenhalgh, Trisha (2005). Diffusion of Innovations in Health Service Organisations: A Systematic Literature (Studies in Urban and Social Change), Blackwell Publishing Professional.
- Greenhalgh T, Potts HWW, Wong G, Bark P, Swinglehurst D (2009). Tensions and paradoxes in electronic patient record research: A systematic literature review using the meta-narrative method. Milbank Quarterly, 87(4), 729-88 (full text; alternate full text)
- Moher D, Tetzlaff J, Tricco AC, Sampson M, Altman DG (2007). Epidemiology and reporting characteristics of systematic reviews. PLoS Med. 4 (3): e78.
- Shojania KG, Sampson M, Ansari MT, Ji J, Doucette S, Moher D (2007). How quickly do systematic reviews go out of date? A survival analysis. Ann. Intern. Med. 147 (4): 224–33.
- Savoie I, Helmer D, Green CJ, Kazanjian A (2003). Beyond Medline: reducing bias through extended systematic review search. Int J Technol Assess Health Care 19 (1): 168–78.
- University of York (Feb 2009), Systematic Reviews: CRD's guidance for undertaking reviews in health care  Retrieved 2009-02-24
- Centre for Reviews and Dissemination
- Cochrane Collaboration
- EPPI-Centre website
- MeSH: Review Literature - articles about the review process
- MeSH: Review [Publication Type] - limit search results to reviews
- PubMed search: "Review Literature"[MAJR]
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