Evidence type
Systematic reviews
Systematic reviews summarise multiple studies using transparent, pre-specified methods. Their strength as evidence depends not only on their process but also on the quality and relevance of the included studies.
What is a systematic review?
A systematic review is a type of research that collects and critically evaluates all available studies on a specific question, using a predefined, transparent method. Instead of just narratively describing the literature, systematic reviews aim to limit bias by following a clear protocol for searching, selecting, and assessing research.
How are systematic reviews performed?
- Researchers define a clear research question.
- They develop criteria for including or excluding studies.
- A comprehensive search of databases is conducted to identify relevant studies.
- Studies are assessed for quality and relevance.
- Findings are summarised, sometimes with statistical analysis (meta-analysis).
How are systematic reviews different from narrative reviews?
Narrative reviews offer an overview of a topic, often with less structured searching and selection of evidence. In contrast, systematic reviews are designed to be more objective and reproducible, using transparent methods for finding and assessing evidence.
Strengths of systematic reviews
- Comprehensive: Aim to include all relevant studies.
- Transparent: Methods are documented, allowing others to see how the review was done.
- Can reduce bias compared to selective or narrative reviews.
- Can sometimes combine data in a meta-analysis to provide more precise estimates of effects.
Limitations and what affects their reliability
- Quality of included studies: If the studies included are of poor quality, the review's conclusions may be weak or misleading.
- Publication bias: Systematic reviews can only summarise published data; missing or unpublished studies may skew results.
- Heterogeneity: Wide differences in study populations, methods, or outcomes can make it hard to combine results meaningfully.
- Protocol deviations or selective reporting: If reviewers do not follow their protocol or selectively report results, the reliability is reduced.
What can systematic reviews "prove"?
While often seen as providing "gold standard" evidence, systematic reviews can only be as reliable as the studies they summarise. A systematic review of poor-quality research will not produce trustworthy conclusions, and even robust reviews can rarely "prove" causation on their own.
Bottom line
Systematic reviews are valuable evidence tools when performed well and based on high-quality primary studies. However, it is important to critically assess how the review was conducted and the nature of the included research before relying on its conclusions.