systematic review 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. a systematic review extracts and interprets data from published studies on the topic, then analyzes, describes, and summarizes interpretations into a refined conclusion. for example, a systematic review of randomized controlled trials is a way of summarizing and implementing evidence-based medicine. a systematic review is the highest form of knowledge in science part of a series onresearch list of academic fields applied sciences r & d formal sciences humanities natural sciences professions social sciences research design research proposal research question writing argument referencing philosophy constructivism empiricism positivism / antipositivism / postpositivism realism critical realism subtle realism research strategy interdisciplinary multimethodology qualitative art-based quantitative methodology action research art methodology critical theory feminism grounded theory hermeneutics historiography narrative inquiry phenomenology pragmatism scientific method methods case study content analysis descriptive statistics discourse analysis ethnography autoethnography experiment field experiment social experiment quasi-experiment field research historical method inferential statistics interviews mapping cultural mapping phenomenography secondary research bibliometrics literature review meta-analysis scoping review systematic review scientific modelling simulation survey tools and software argument technology geographic information system software library and information science software bibliometrics reference management science software qualitative data analysis simulation statistics philosophy portal 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. a systematic review may examine clinical tests, public health interventions, environmental interventions, social interventions, adverse effects, qualitative evidence syntheses, methodological reviews, policy reviews, and economic evaluations. an understanding of systematic reviews and how to implement them in practice is common for professionals in health care, public health, and public policy. characteristics a systematic review can be designed to provide a thorough summary of current literature relevant to a research question. 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. 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. as evidence rating can be subjective, multiple people may be consulted to resolve any scoring differences between how evidence is rated. the eppi-centre, cochrane and the joanna briggs institute have all been influential in developing methods for combining both qualitative and quantitative research in systematic reviews. 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 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. 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. a list of prisma guideline extensions is hosted by the equator (enhancing the quality and transparency of health research) network. 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; and emerge (improving reporting of meta-ethnography) for meta-ethnograph. 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. types there are over 30 types of systematic review and table 1 below summarises some of these, but it is not exhaustive. it is important to note that there is not always consensus on the boundaries and distinctions between the approaches described below. table 1: a summary of some of the types of systematic review review type summary mapping review/systematic map a mapping review maps existing literature and categorizes data. the method characterizes quantity and quality of literature, including by study design and other features. mapping reviews can be used to identify the need for primary or secondary research. meta-analysis a meta-analysis is a statistical analysis that combines the results of multiple quantitative studies. using statistical methods, results are combined to provide evidence from multiple studies. the two types of data generally used for meta-analysis in health research are individual participant data and aggregate data (such as odds ratios or relative risks). mixed studies review/mixed methods review refers to any combination of methods where one significant stage is a literature review (often systematic). it can also refer to a combination of review approaches such as combining quantitative with qualitative research. qualitative systematic review/qualitative evidence synthesis this method for integrates or compares findings from qualitative studies. the method can include 'coding' the data and looking for 'themes' or 'constructs' across studies. multiple authors may improve the 'validity' of the data by potentially reducing individual bias. rapid review an assessment of what is already known about a policy or practice issue, which uses systematic review methods to search for and critically appraise existing research. rapid reviews are still a systematic review, however parts of the process may be simplified or omitted in order to increase rapidity. rapid reviews were used during the covid-19 pandemic. systematic review a systematic search for data, using a repeatable method. it includes appraising the data (for example the quality of the data) and a synthesis of research data. systematic search and review combines methods from a 'critical review' with a comprehensive search process. this review type is usually used to address broad questions to produce the most appropriate evidence synthesis. this method may or may not include quality assessment of data sources. systematized review include elements of systematic review process, but searching is often not as comprehensive as a systematic review and may not include quality assessments of data sources. scoping reviews scoping reviews are distinct from systematic reviews in several important ways. a scoping review is an attempt to search for concepts by mapping the language and data which surrounds those concepts and adjusting the search method iteratively to synthesize evidence and assess the scope of an area of inquiry. this can mean that the concept search and method (including data extraction, organisation and analysis) are refined throughout the process, sometimes requiring deviations from any protocol or original research plan. a scoping review may often be a preliminary stage before a systematic review, which 'scopes' out an area of inquiry and maps the language and key concepts to determine if a systematic review is possible or appropriate, or to lay the groundwork for a full systematic review. the goal can be to assess how much data or evidence is available regarding a certain area of interest. this process is further complicated if it is mapping concepts across multiple languages or cultures. as a scoping review should be systematically conducted and reported (with a transparent and repeatable method), some academic publishers categorize them as a kind of 'systematic review', which may cause confusion. scoping reviews are helpful when it is not possible to carry out a systematic synthesis of research findings, for example, when there are no published clinical trials in the area of inquiry. scoping reviews are helpful when determining if it is possible or appropriate to carry out a systematic review, and are a useful method when an area of inquiry is very broad, for example, exploring how the public are involved in all stages systematic reviews. there is still a lack of clarity when defining the exact method of a scoping review as it is both an iterative process and is still relatively new. there have been several attempts to improve the standardisation of the method, for example via a prisma guideline extension for scoping reviews (prisma-scr). prospero (the international prospective register of systematic reviews) does not permit the submission of protocols of scoping reviews, although some journals will publish protocols for scoping reviews. stages while there are multiple kinds of systematic review methods, the main stages of a review can be summarised as follows: defining the research question defining an answerable question and agreeing an objective method is required to design a useful systematic review. best practice recommends publishing the protocol of the review before initiating it to reduce the risk of unplanned research duplication and to enable transparency, and consistency between methodology and protocol. clinical reviews of quantitative data are often structured using the acronym pico, which stands for 'population or problem', 'intervention or exposure', 'comparison' and 'outcome', with other variations existing for other kinds of research. for qualitative reviews pico is 'population or problem', 'interest' and 'context'. searching for relevant data sources planning how the review will search for relevant data from research that matches certain criteria is a decisive stage in developing a rigorous systematic review. relevant criteria can include only selecting research that is good quality and answers the defined question. the search strategy should be designed to retrieve literature that matches the protocol's specified inclusion and exclusion criteria. the methodology section of a systematic review should list all of the databases and citation indices that were searched. the titles and abstracts of identified articles can be checked against pre-determined criteria for eligibility and relevance. each included study may be assigned an objective assessment of methodological quality, preferably by using methods conforming to the preferred reporting items for systematic reviews and meta-analyses (prisma) statement, or the high-quality standards of cochrane. common information sources used in searches include scholarly databases of peer-reviewed articles such as medline, web of science, embase, and pubmed as well as sources of unpublished literature such as clinical trial registries and grey literature collections. key references can also be yielded through additional methods such as citation searching, reference list checking (related to a search method called 'pearl growing'), manually searching information sources not indexed in the major electronic databases (sometimes called 'hand-searching'), and directly contacting experts in the field. to be systematic, searchers must use a combination of search skills and tools such as database subject headings, keyword searching, boolean operators, proximity searching, while attempting to balance the sensitivity (systematicity) and precision (accuracy). inviting and involving an experienced information professional or librarian can notably improve the quality of systematic review search strategies and reporting. 'extraction' of relevant data a visualisation of data being 'extracted' and 'combined' in a cochrane intervention effect review where a meta-analysis is possible relevant data are 'extracted' from the data sources according to the review method. it is important to note that the data extraction method is specific to the kind of data, and data extracted on 'outcomes' is only relevant to certain types of reviews. for example, a systematic review of clinical trials might extract data about how the research was done (often called the method or 'intervention'), who participated in the research (including how many people), how it was paid for (for example funding sources) and what happened (the outcomes). effectively, relevant data being extracted and 'combined' in a cochrane intervention effect review, where a meta-analysis is possible. assess the eligibility of the data this stage involves assessing the eligibility of data for inclusion in the review, by judging it against criteria identified at the first stage. this can include assessing if a data source meets the eligibility criteria, and recording why decisions about inclusion or exclusion in the review were made. software can be used to support the selection process including text mining tools and machine learning, which can automate aspects of the process. the 'systematic review toolbox' is a community driven, web-based catalogue of tools, to help reviewers chose appropriate tools for reviews. analyse and combine the data analysing and combining data can provide an overall result from all the data. because this combined result may use qualitative or quantitative data from all eligible sources of