# Evidence synthesis: choosing between meta-analysis and narrative synthesis

> Chapter 6 of 8 of The Systematic Review E-book by AIPRA. Evidence synthesis is the decision about how to bring extracted data together. Meta-analysis is appropriate when included studies are sufficiently similar in their PICO elements for a pooled estimate to be meaningful, and when heterogeneity is not so extreme that a single average would mislead readers. Narrative synthesis is indicated when studies differ too much in design, population, or intervention, or when outcome data are too incompletely reported for defensible pooling. Narrative synthesis should follow SWiM (Synthesis Without Meta-analysis) reporting guidance so it remains rigorous, transparent, and reproducible rather than subjective commentary.

- Source: https://aipra.co/systematic-review-ebook/evidence-synthesis
- Topics: evidence synthesis; narrative synthesis vs meta analysis; SWiM guideline; synthesis without meta-analysis; when is meta-analysis appropriate
- Last updated: 2026-09-05
- Licence: free to read; cite as shown at the end of this file.

## Key points

- The choice between meta-analysis and narrative synthesis depends on conceptual and statistical similarity of the included studies, not on preference.
- Meta-analysis requires sufficient similarity across PICO elements for a pooled summary to be clinically meaningful.
- Heterogeneity thresholds such as I-squared are rules of thumb; the Cochrane Handbook stresses interpreting heterogeneity in context rather than against a single cut-off.
- Narrative synthesis is the honest choice when designs, populations, or interventions differ too much, or when outcome reporting is too incomplete to pool.
- Narrative synthesis should follow SWiM reporting guidance so it stays structured, transparent, and auditable.

## Chapter outline

1. What evidence synthesis decides
2. Choosing an appropriate synthesis method
3. How reviewers usually decide
4. Planning synthesis in the workflow

## Questions this chapter answers

### When is meta-analysis appropriate in a systematic review?

Meta-analysis is appropriate when included studies are similar enough in population, intervention, comparator, and outcome that a pooled estimate is clinically meaningful, and when between-study heterogeneity is not so large that an average effect would mislead readers. Both conceptual and statistical similarity matter.

### What is the difference between narrative synthesis and meta-analysis?

Meta-analysis statistically pools effect estimates into a summary effect with a confidence interval. Narrative synthesis describes and structures findings without mathematical pooling, and is used when studies are too heterogeneous or too incompletely reported for pooling to be defensible.

### What is the SWiM guideline?

SWiM stands for Synthesis Without Meta-analysis. It is reporting guidance for systematic reviews that synthesize results without statistical pooling, and it exists to keep narrative synthesis rigorous, transparent, and reproducible rather than subjective.

## References cited

- Campbell M, McKenzie JE, Sowden A, et al. Synthesis without meta-analysis (SWiM) in systematic reviews: reporting guideline. BMJ. 2020;368:l6890. doi:10.1136/bmj.l6890
- Higgins JPT, Thompson SG, Deeks JJ, Altman DG. Measuring inconsistency in meta-analyses. BMJ. 2003;327(7414):557-560. doi:10.1136/bmj.327.7414.557

## How to cite this chapter

AIPRA. Evidence synthesis: choosing between meta-analysis and narrative synthesis. In: The Systematic Review E-book. Updated 2026-09-05. Available from: https://aipra.co/systematic-review-ebook/evidence-synthesis