A systematic literature review is a research methodology designed to answer a focused research question. Authors conduct a methodical and comprehensive literature synthesis focused on a well-formulated research question. Its aim is to identify and synthesize all of the scholarly research on a particular topic, including both published and unpublished studies. Systematic reviews are conducted in an unbiased, reproducible way to provide evidence for practice and policy-making and identify gaps in research. Every step of the review, including the search, must be documented for reproducibility.
Researchers in medicine may be most familiar with Cochrane Reviews, which synthesize randomized controlled trials to evaluate specific medical interventions. Systematic reviews are conducted in many other fields, though the type of evidence analyzed varies with the research question.
Systematic reviews require more time and manpower than traditional literature reviews. Before beginning a systematic review, researchers should address these questions:
Is there is enough literature published on the topic to warrant a review?
Systematic reviews are designed to distill the evidence from many studies into actionable insights. Is there a body of evidence available to analyze, or does more primary research need to be done?
Can your research question be answered by a systematic review?
Systematic review questions should be specific and clearly defined. Questions that fit the PICO (problem/patient, intervention, comparison, outcome) format are usually well-suited for the systematic review methodology. The research question determines the search strategy, inclusion criteria, and data that you extract from the selected studies, so it should be clearly defined at the start of the review process.
Do you have a protocol outlining the review plan?
The protocol is the roadmap for the review project. A good protocol outlines study methodology, includes the rationale for the systematic review, and describes the key question broken into PICO components. It is also a good place to plan out inclusion/exclusion criteria, databases that will be searched, data abstraction and management methods, and how the studies will be assessed for methodological quality.
Do you have a team of experts?
A systematic review is team effort. Having multiple reviewers minimizes bias and strengthens analysis. Teams are often composed of subject experts, two or more literature screeners, a librarian to conduct the search, and a statistician to analyze the data.
Do you have the time that it takes to properly conduct a systematic review?
Systematic reviews typically take 12-18 months.
Do you have a method for discerning bias?
There are many types of bias, including selection, performance, & reporting bias, and assessing the risk of bias of individual studies is an important part of your study design.
Can you afford to have articles in languages other than English translated?
You should include all relevant studies in your systematic review, regardless of the language they were published in, so as to avoid language bias.
If your project does not meet the above criteria, there are many more options for conducting a synthesis of the literature. The chart below highlights several review methodologies. Reproduced from: Grant MJ, Booth A. A typology of reviews: an analysis of 14 review types and associated methodologies. Health Info Libr J. 2009 Jun;26(2):91-108. doi: 10.1111/j.1471-1842.2009.00848.x . Review. PubMed PMID: 19490148
|Critical review||Aims to demonstrate writer has extensively researched literature and critically evaluated its quality. Goes beyond mere description to include degree of analysis and conceptual innovation. Typically results in hypothesis or model.||Seeks to identify significant items in the field.||No formal quality assessment. Attempts to evaluate according to contribution.||Typically narrative, perhaps conceptual or chronological.||Significant component: seeks to identify conceptual contribution to embody existing or derive new theory.|
|Literature review||Generic term: a search for published materials that provide examination of recent or current literature. Can cover wide range of subjects at various levels of completeness and comprehensiveness. May include research findings.||May or may not include comprehensive searching.||May or may not include quality assessment.||Typically narrative.||Analysis may be chronological, conceptual, thematic, etc.|
|Mapping review/systematic map||Maps out and categorizes existing literature from which to commission further reviews and/or primary research by identifying gaps in research literature.||Completeness of searching determined by time/scope constraints.||No formal quality assessment.||May be graphical and tabular.||Characterizes quantity and quality of literature, perhaps by study design and other key features. May identify need for primary or secondary research.|
|Meta-analysis||Technique that statistically combines the results of quantitative studies to provide a more precise effect of the results.||Aims for exhaustive searching. May use funnel plot to assess completeness.||Quality assessment may determine inclusion/exclusion and/or sensitivity analyses.||Graphical and tabular with narrative commentary.||Numerical analysis of measures of effect assuming absence of heterogeneity.|
|Mixed studies review/mixed methods review||Refers to any combination of methods where one significant component is a literature review (usually systematic). Within a review context it refers to a combination of review approaches for example combining quantitative with qualitative research or outcome with process studies.||Requires either very sensitive search to retrieve all studies or separately conceived quantitative and qualitative strategies.||Requires either a generic appraisal instrument or separate appraisal processes with corresponding checklists.||Typically both components will be presented as narrative and in tables. May also employ graphical means of integrating quantitative and qualitative studies.||Analysis may characterize both quantitative and qualitative studies and look for correlations between their characteristics or use gap analysis to identify aspects present in one type of study but missing in the other.|
|Overview||Generic term: summary of the [medical] literature that attempts to survey the literature and describe its characteristics.||May or may not include comprehensive searching (depends whether systematic overview or not).||May or may not include quality assessment (depends whether systematic overview or not).||Synthesis depends on whether systematic overview or not. Typically narrative but may include tabular features.||Analysis may be chronological, conceptual, thematic, etc.|
|Qualitative systematic review/qualitative evidence synthesis||Method for integrating or comparing the findings from qualitative studies. It looks for ‘themes’ or ‘constructs’ that lie in or across individual qualitative studies.||May employ selective or purposive sampling.||Quality assessment typically used to mediate messages not for inclusion/exclusion.||Qualitative, narrative synthesis.||Thematic analysis, may include conceptual models.|
|Rapid review||Assessment of what is already known about a policy or practice issue, by using systematic review methods to search and critically appraise existing research.||Completeness of searching determined by time constraints.||Time-limited formal quality assessment.||Typically narrative and tabular.||Quantities of literature and overall quality/direction of effect of literature.|
|Scoping review||Preliminary assessment of potential size and scope of available research literature. Aims to identify nature and extent of research evidence (usually including ongoing research).||Completeness of searching determined by time/scope constraints. May include research in progress.||No formal quality assessment.||Typically tabular with some narrative commentary.||Characterizes quantity and quality of literature, perhaps by study design and other key features. Attempts to specify a viable review.|
|State-of-the-art review||Tend to address more current matters in contrast to other combined retrospective and current approaches. May offer new perspectives on issue or point out area for further research.||Aims for comprehensive searching of current literature.||No formal quality assessment.||Typically narrative, may have tabular accompaniment.||Current state of knowledge and priorities for future investigation and research.|
|Systematic review||Seeks to systematically search for, appraise and synthesize research evidence, often adhering to guidelines on the conduct of a review.||Aims for exhaustive, comprehensive searching.||Quality assessment may determine inclusion/exclusion.||Typically narrative with tabular accompaniment.||What is known; recommendations for practice. What remains unknown; uncertainty around findings, recommendations for future research.|
|Systematic search and review||Combines strengths of critical review with a comprehensive search process. Typically addresses broad questions to produce ‘best evidence synthesis.'||Aims for exhaustive, comprehensive searching.||May or may not include quality assessment.||Minimal narrative, tabular summary of studies.||What is known; recommendations for practice. Limitations.|
|Systematized review||Attempt to include elements of systematic review process while stopping short of systematic review. Typically conducted as postgraduate student assignment.||May or may not include comprehensive searching.||May or may not include quality assessment.||Typically narrative with tabular accompaniment.||What is known; uncertainty around findings; limitations of methodology.|
|Umbrella review||Specifically refers to review compiling evidence from multiple reviews into one accessible and usable document. Focuses on broad condition or problem for which there are competing interventions and highlights reviews that address these interventions and their results.||Identification of component reviews, but no search for primary studies.||Quality assessment of studies within component reviews and/or of reviews themselves.||Graphical and tabular with narrative commentary.||What is known; recommendations for practice. What remains unknown; recommendations for future research.|