A systematic review is a literature review focused on a research 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 aims to provide an exhaustive summary of literature relevant to a research question. The first step of a systematic review is a thorough search of the literature for relevant papers. The Methodology section of the review will list the databases and citation indexes searched, such as Web of Science and PubMed, as well as any individual journals. Next, the titles and the abstracts of the identified articles are checked against pre-determined criteria for eligibility and relevance. Each paper may be assigned an objective assessment of methodological quality using the Jadad scale or similar rating system. Systematic reviews often, but not always, uses statistical techniques (meta-analysis) to combine results of the eligible studies, or at least uses scoring of the levels of evidence depending on the methodology used. Systematic review is often applied in the biomedical or healthcare context, but it can be applied in any field of research. Groups like the Campbell Collaboration are promoting the use of systematic reviews in policy-making beyond just healthcare.
A systematic review uses an objective and transparent approach for research synthesis, with the aim of minimizing 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 18,000 specialists in health care who systematically review randomised trials of the effects of prevention, treatments and rehabilitation as well as health systems interventions. When appropriate, they also include the results of other types of research. Cochrane reviews are published in The Cochrane Database of Systematic Reviews section of The Cochrane Library, which to date (July 2010) includes 6244 articles: 4130 reviews; and 1911 protocols for additional reviews being conducted. The 2009 impact factor for The Cochrane Library is 5.653 and it is ranked 11th in the “Medicine, General & Internal” category http://www.thecochranelibrary.com/view/0/AboutTheCochraneLibrary.html.
The Cochrane Group provides a handbook for systematic reviewers of interventions, where they suggest that each systematic review should contain the following main sections:
There are seven steps for preparing and maintaining a systematic review, as outlined in the Cochrane Handbook:
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.