Lesson 35 of 36 · 15 min
Systematic reviews & meta-analysis
A systematic review asks a predefined question and searches for the relevant evidence using explicit methods. A meta-analysis may statistically combine results, but a pooled number is only as interpretable as the studies being pooled.
Review vs meta-analysis
Systematic search and statistical pooling are related but different.
Systematic review
Uses explicit eligibility, search, extraction and appraisal methods to summarize a body of evidence.
Meta-analysis
Statistically combines compatible effect estimates when pooling is appropriate.
Heterogeneity
Studies can disagree because they are studying different things.
Population, formulation, route, dose, duration and endpoint differences can produce genuine heterogeneity. Statistical heterogeneity is not simply a nuisance to average away.
A pooled estimate across biologically or clinically incompatible studies may look precise while answering a question nobody actually asked.
Garbage in, precision out
A meta-analysis cannot repair biased primary studies.
Pooling several small or high-risk-of-bias studies can produce a narrow-looking estimate without eliminating the problems shared by those studies.
Read the inclusion criteria, risk-of-bias assessment and sensitivity analyses before treating the pooled number as stronger than every component study.
Synthesis rule
More studies is not automatically better evidence.
The question is whether the included studies are relevant, trustworthy and similar enough for the synthesis to mean something.
Key takeaways
- ✓A systematic review is a method of evidence synthesis; a meta-analysis is a statistical technique that may be part of it.
- ✓Study eligibility and search methods determine what enters the evidence base.
- ✓Heterogeneity can reflect real biological and clinical differences.
- ✓Pooling does not remove bias from poor primary studies.
- ✓Sensitivity analyses and risk-of-bias judgments matter when interpreting a pooled result.
Further reading
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