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Its aim is to identify and synthesize all of the scholarly research on a particular topic, including both published and unpublished studies.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 extraction 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 for a systematic review, there are many more options for conducting a synthesis of the literature. We recommend using the Right Review Tool to determine if your team is selecting the appropriate evidence synthesis project. The table below highlights several common review methodologies.
|
Label |
Description |
Search |
Appraisal |
Synthesis |
Analysis |
|---|---|---|---|---|---|
| Literature/Narrative 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. |
| 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. |
| 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. |
| 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. |
| 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. |