A systematic review (also systematic literature review or structured literature review, SLR) 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. Besides health interventions, systematic reviews may concern clinical tests, public health interventions, social interventions, adverse effects, and economic evaluations. Systematic reviews are not limited to medicine and are quite common in all other sciences where data are collected, published in the literature, and an assessment of methodological quality for a precisely defined subject would be helpful.
A systematic review aims to provide an exhaustive summary of current 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, Embase, and PubMed, as well as any hand-searched individual journals. Next, the titles and the abstracts of the identified articles are checked against pre-determined criteria for eligibility and relevance. This list will always depend on the research problem. Each included study may be assigned an objective assessment of methodological quality preferably using a method conforming to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement (the current guideline) or the high quality standards of Cochrane collaboration.
Systematic reviews often, but not always, use statistical techniques (meta-analysis) to combine results of the eligible studies, or at least use scoring of the levels of evidence depending on the methodology used. An additional rater may be consulted to resolve any scoring differences between raters. 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 has been influential in developing methods for combining both qualitative and quantitative research in systematic reviews.
Recent developments in systematic reviews include realist reviews, and the meta-narrative approach. These approaches try to overcome the problems of methodological and epistemological heterogeneity in the diverse literatures existing on some subjects. The PRISMA statement suggests a standardized way to ensure a transparent and complete reporting of systematic reviews, and is now required for this kind of research by more than 170 medical journals worldwide.
The Cochrane Collaboration is a group of over 31,000 specialists in healthcare 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. The 2010 impact factor for The Cochrane Database of Systematic Reviews was 6.186, and it was ranked 10th in the “Medicine, General & Internal” category. There are six types of Cochrane Review:
- Intervention reviews assess the benefits and harms of interventions used in healthcare and health policy.
- Diagnostic test accuracy reviews assess how well a diagnostic test performs in diagnosing and detecting a particular disease.
- Methodology reviews address issues relevant to how systematic reviews and clinical trials are conducted and reported.
- Qualitative reviews synthesize qualitative evidence to address questions on aspects other than effectiveness.
- Prognosis reviews address the probable course or future outcome(s) of people with a health problem.
- Overviews of Systematic Reviews (OoRs) are a new type of study in order to compile multiple evidence from systematic reviews into a single document that is accessible and useful to serve as a friendly front end for the Cochrane Collaboration with regard to healthcare decision-making.
The Cochrane Collaboration provides a handbook for systematic reviewers of interventions which "provides guidance to authors for the preparation of Cochrane Intervention reviews." The Cochrane Handbook outlines eight general steps for preparing a systematic review:
- Defining the review question(s) and developing criteria for including studies
- Searching for studies
- Selecting studies and collecting data
- Assessing risk of bias in included studies
- Analysing data and undertaking meta-analyses
- Addressing reporting biases
- Presenting results and "summary of findings" tables
- Interpreting results and drawing conclusions
The Cochrane Handbook forms the basis of two sets of standards for the conduct and reporting of Cochrane Intervention Reviews (MECIR - Methodological Expectations of Cochrane Intervention 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 can be improved by a universally agreed upon set of standards and guidelines. A further study by the same group found that of 100 systematic reviews monitored, 7% needed updating at the time of publication, another 4% within a year, and another 11% within 2 years; this figure was higher in rapidly changing fields of medicine, especially cardiovascular medicine. A 2003 study suggested that extending searches beyond major databases, perhaps into grey literature, would increase the effectiveness of reviews.
Roberts and colleagues highlighted the problems with systematic reviews, particularly those conducted by the Cochrane Collaboration, noting that published reviews are often biased, out of date and excessively long. They criticized Cochrane reviews as not being sufficiently critical in the selection of trials and including too many of low quality. They proposed several solutions, including limiting studies in meta-analyses and reviews to registered clinical trials, requiring that original data be made available for statistical checking (a proposal that had been previously made by other researchers), paying greater attention to sample size estimates, and eliminating dependence on only published data.
Some of these difficulties were noted early on as described by Altman: "much poor research arises because researchers feel compelled for career reasons to carry out research that they are ill equipped to perform, and nobody stops them." Methodological limitations of meta-analysis have also been noted. Another concern is that the methods used to conduct a systematic review are sometimes changed once researchers see the available trials they are going to include. Bloggers have described retractions of systematic reviews and published reports of studies included in published systematic reviews.
Systematic reviews are increasingly prevalent in other fields, such as international development research. Subsequently, a number of donors – most notably the UK Department for International Development (DFID) and AusAid – are focusing more attention and resources on testing the appropriateness of systematic reviews in assessing the impacts of development and humanitarian interventions.
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- Centre for Reviews and Dissemination, University of York.
- Cochrane Collaboration
- Evidence for Policy and Practice Information and Co-ordinating Centre (EPPI-Centre), University of London
- MeSH: Review Literature—articles about the review process
- MeSH: Review [Publication Type] - limit search results to reviews
- PubMed search: "Review Literature" [MAJR]
- Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) Statement, "an evidence-based minimum set of items for reporting in systematic reviews and meta-analyses"