Explainer · Plain-language
What is a systematic review?
A systematic review is a form of evidence synthesis that uses explicit, pre-defined, reproducible methods to identify, appraise, and combine all the relevant studies that address a focused research question. It sits at the top of the evidence hierarchy for answering questions about effectiveness.
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What makes a review "systematic"
Unlike a traditional narrative review, a systematic review follows pre-specified, documented methods so that it is transparent and reproducible. It defines a focused question, sets explicit inclusion and exclusion criteria, runs a comprehensive search across multiple databases, screens results against the criteria, appraises the included studies for risk of bias, and synthesises the findings — minimising bias at each step.
Protocol and registration
A systematic review begins with a protocol that specifies the question, search strategy, eligibility criteria, and analysis plan before the review is carried out. Registering the protocol — for health reviews, commonly in PROSPERO — guards against selective reporting and duplication, and lets others see what was planned in advance.
Synthesis and meta-analysis
Once studies are selected and appraised, their results are synthesised. This may be narrative, or, where studies are similar enough, quantitative — a meta-analysis that statistically combines effect estimates into a pooled result. Not every systematic review includes a meta-analysis; the decision depends on whether combining the studies is methodologically appropriate.
Reporting with PRISMA
Systematic reviews are reported following PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses), most recently the PRISMA 2020 statement. PRISMA provides a checklist and a flow diagram showing how records moved from search results to included studies, so readers can judge the review's completeness and rigour. A related extension, PRISMA-ScR, covers scoping reviews.
Key facts
At a glance
- Aim: answer a focused question via evidence synthesis
- Method: pre-defined protocol, comprehensive search, appraisal, synthesis
- Registration: PROSPERO (for health/social-care reviews)
- Reporting: PRISMA 2020 statement and flow diagram
- May include: a meta-analysis (when studies are combinable)
- Contrast: broader-mapping scoping reviews use PRISMA-ScR
Common misconceptions
What people often get wrong
Often heard: A systematic review is just a thorough literature review.
Actually: No — a systematic review follows an explicit, pre-registered, reproducible protocol with defined criteria and appraisal, which a traditional narrative review does not. The method, not just the thoroughness, is what makes it systematic.
Often heard: Every systematic review contains a meta-analysis.
Actually: No — a meta-analysis is only included when it is appropriate to combine the studies statistically. Many systematic reviews synthesise findings narratively without pooling the data.
Going deeper
