Systematic review
A systematic review is a scholarly synthesis of the evidence on a clearly presented topic using critical methods to identify, define and assess research on the topic.[1] A systematic review extracts and interprets data from published studies on the topic (in the scientific literature), then analyzes, describes, critically appraises and summarizes interpretations into a refined evidence-based conclusion.[1][2] For example, a systematic review of randomized controlled trials is a way of summarizing and implementing evidence-based medicine.[3]
For the academic journal, see Systematic Reviews (journal).
While a systematic review may be applied in the biomedical or health care context, it may also be used where an assessment of a precisely defined subject can advance understanding in a field of research.[4] A systematic review may examine clinical tests, public health interventions, environmental interventions,[5] social interventions, adverse effects, qualitative evidence syntheses, methodological reviews, policy reviews, and economic evaluations.[6][7]
Systematic reviews are closely related to meta-analyses, and often the same instance will combine both (being published with a subtitle of "a systematic review and meta-analysis"). The distinction between the two is that a meta-analysis uses statistical methods to induce a single number from the pooled data set (such as an effect size), whereas the strict definition of a systematic review excludes that step. However, in practice, when one is mentioned the other may often be involved, as it takes a systematic review to assemble the information that a meta-analysis analyzes, and people sometimes refer to an instance as a systematic review even if it includes the meta-analytical component.
An understanding of systematic reviews and how to implement them in practice is common for professionals in health care, public health, and public policy.[1]
Systematic reviews contrast with a type of review often called a narrative review. Systematic reviews and narrative reviews both review the literature (the scientific literature), but the term literature review without further specification refers to a narrative review.
Characteristics[edit]
A systematic review can be designed to provide a thorough summary of current literature relevant to a research question.[1] A systematic review uses a rigorous and transparent approach for research synthesis, with the aim of assessing and, where possible, minimizing bias in the findings. While many systematic reviews are based on an explicit quantitative meta-analysis of available data, there are also qualitative reviews and other types of mixed-methods reviews which adhere to standards for gathering, analyzing and reporting evidence.[8]
Systematic reviews of quantitative data or mixed-method reviews sometimes use statistical techniques (meta-analysis) to combine results of eligible studies. Scoring levels are sometimes used to rate the quality of the evidence depending on the methodology used, although this is discouraged by the Cochrane Library.[9] As evidence rating can be subjective, multiple people may be consulted to resolve any scoring differences between how evidence is rated.[10][11][12]
The EPPI-Centre, Cochrane, and the Joanna Briggs Institute have been influential in developing methods for combining both qualitative and quantitative research in systematic reviews.[13][14][15] Several reporting guidelines exist to standardise reporting about how systematic reviews are conducted. Such reporting guidelines are not quality assessment or appraisal tools. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement[16] 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.[17] Several specialized PRISMA guideline extensions have been developed to support particular types of studies or aspects of the review process, including PRISMA-P for review protocols and PRISMA-ScR for scoping reviews.[17] A list of PRISMA guideline extensions is hosted by the EQUATOR (Enhancing the QUAlity and Transparency Of health Research) Network.[18] However, the PRISMA guidelines have been found to be limited to intervention research and the guidelines have to be changed in order to fit non-intervention research. As a result, Non-Interventional, Reproducible, and Open (NIRO) Systematic Reviews was created to counter this limitation.[19]
For qualitative reviews, reporting guidelines include ENTREQ (Enhancing transparency in reporting the synthesis of qualitative research) for qualitative evidence syntheses; RAMESES (Realist And MEta-narrative Evidence Syntheses: Evolving Standards) for meta-narrative and realist reviews;[20][21] and eMERGe (Improving reporting of Meta-Ethnography) for meta-ethnograph.[13]
Developments in systematic reviews during the 21st century included realist reviews and the meta-narrative approach, both of which addressed problems of variation in methods and heterogeneity existing on some subjects.[22][23]
Automation[edit]
Living systematic reviews are a newer kind of semi-automated, up-to-date online summaries of research that are updated as new research becomes available.[53] The difference between a living systematic review and a conventional systematic review is the publication format. Living systematic reviews are "dynamic, persistent, online-only evidence summaries, which are updated rapidly and frequently".[54]
The automation or semi-automation of the systematic process itself is increasingly being explored. While little evidence exists to demonstrate it is as accurate or involves less manual effort, efforts that promote training and using artificial intelligence for the process are increasing.[55][53]
A 2022 publication identified 24 systematic review tools and ranked them by inclusion of 30 features deemed most important when performing a systematic review in accordance with best practices. The top six software tools (with at least 21/30 key features) are all proprietary paid platforms, typically web-based, and include:[86]
The Cochrane Collaboration provides a handbook for systematic reviewers of interventions which "provides guidance to authors for the preparation of Cochrane Intervention reviews."[37] The Cochrane Handbook also outlines steps for preparing a systematic review[37] and forms the basis of two sets of standards for the conduct and reporting of Cochrane Intervention Reviews (MECIR; Methodological Expectations of Cochrane Intervention Reviews).[87] It also contains guidance on integrating patient-reported outcomes into reviews.
Limitations[edit]
Out-dated or risk of bias[edit]
While systematic reviews are regarded as the strongest form of evidence, a 2003 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.[88] 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.[89] A 2003 study suggested that extending searches beyond major databases, perhaps into grey literature, would increase the effectiveness of reviews.[90]
Some authors have highlighted problems with systematic reviews, particularly those conducted by Cochrane, noting that published reviews are often biased, out of date, and excessively long.[91] Cochrane reviews have been criticized 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, paying greater attention to sample size estimates, and eliminating dependence on only published data. Some of these difficulties were noted as early as 1994: