What is a systematic review?
A systematic review summarises the available research on a clearly defined question using predefined, transparent rules. It ranks high in the evidence hierarchy because it does not simply pick out a few fitting studies but searches and appraises them systematically.
Systematic review explained simply
Suppose you want to know whether a substance helps with a complaint. Single studies are often small, designed differently and contradict each other. A systematic review therefore collects all relevant studies and appraises them against criteria set in advance.
Typical steps:
- Define the question (who? which intervention? which comparison? which outcome?).
- Search strategy across several databases, so nothing is selectively missed.
- Apply inclusion and exclusion criteria, usually by two people independently.
- Assess quality, for example the risk of bias.
- Synthesise results, either descriptively or statistically.
Systematic review and meta-analysis
When the numbers of several studies are pooled mathematically, it is called a meta-analysis. It can be part of a systematic review, but not every review contains one. If the studies are too different, findings are only described.
How a review compares to other study types
Where a systematic review fits
| Type | What is examined? | Strength of evidence |
|---|---|---|
| Type Single randomised controlled trial | What is examined? One intervention in one group, with a comparison | Strength of evidence High, but limited to that one study |
| Type Traditional narrative review | What is examined? Selected studies without a fixed method | Strength of evidence Can be one-sided |
| Type Systematic review | What is examined? All matching studies per a fixed protocol | Strength of evidence High, depends on the quality of the included studies |
What reviews show about turmeric – and what they do not
In the debate on curcumin and joint complaints, systematic reviews are often cited. They report that in some smaller studies pain reduction with curcumin extracts was comparable to common painkillers. The caveat reviews themselves state is important: many included studies were small, short and dosed differently, with non-uniform products. A review is only as good as the studies it contains.
How to read a review critically
- How many studies and participants are included?
- How high is the risk of bias in the individual studies?
- Were products and doses comparable?
- Are there conflicts of interest, such as manufacturer funding?
The overall assessment is called evidence. This text is not medical advice.
Limits of systematic reviews
Even a review can be biased. If negative studies are never published (publication bias), they are missing from the analysis. Reviews also age: new studies appear after publication. Check the publication year and ask whether the question fits your situation, such as age, condition and dose.
Frequently asked questions
Which is better: a systematic review or a single study? A well-done review is usually more informative than a single study because it pools more data. It can only be as reliable as the studies it contains.
How can I tell a review is truly systematic? There is a protocol set in advance, the search strategy is disclosed and inclusion and exclusion criteria are stated. The PRISMA checklist is often used for this.
What is the Cochrane Collaboration? An international organisation that produces systematic reviews on medical questions and publishes them to strict standards.
