[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Epidemiology](https://durgical.com/en/epidemiology/) › [Evidence-based medicine](https://durgical.com/en/epidemiology/areas/ebm/) › Forest plot

# Forest plot

- Concept

A forest plot shows each study as a square with its interval, and the pooled effect as a diamond, against the line of no effect.

Updated Nov 5, 2026

## Key points

- Square: each study's effect; its area is proportional to the weight.
- Horizontal line: the study's 95 % CI; if it crosses the line of no effect, that study alone is not significant.
- Diamond: pooled effect; if it does not touch the line of no effect, the pooled result is statistically significant.
- Line of no effect: 1 for RR, OR and HR (on a log scale); 0 for mean or risk differences.
- Below the plot, heterogeneity (I²) and the test for the overall effect are reported.
- Check which side shows benefit: the axis label says which side favours each group.

## What it is

The forest plot summarises a meta-analysis. Each row is a study: the square marks its effect and its size shows the weight; the horizontal line is its confidence interval. The diamond at the bottom is the pooled effect: its centre is the estimate and its width the CI. The vertical line of no effect sits at 1 for ratios (RR, OR, HR) and at 0 for differences.

## Worked example

- Data (made up): 4 trials of a falls-prevention programme; RR (95 % CI): 0.70 (0.50–0.98); 0.85 (0.60–1.20); 0.60 (0.40–0.90); 1.05 (0.70–1.58).
- Trials 1 and 3: their CI lies entirely below 1: a significant reduction in each.
- Trials 2 and 4: their CI crosses 1: on their own they show no difference.
- Inverse-variance weights on the log scale: ≈ 30 %, 28 %, 21 % and 21 %: the first two, with narrower CIs, weigh more.
- Diamond (fixed effect): RR 0.78 (95 % CI 0.65 to 0.94); it does not touch the line at 1.
- Relative reduction: 1 − 0.78 = 0.22, that is, 22 % fewer falls.
- Reading: two inconclusive trials do not cancel the set; combined, the 4 show a benefit with a CI that excludes 1.

## Markers

- Square
- CI
- Diamond
- Line of no effect
- I²

## In practice

- Look first at the diamond and the line of no effect, then at the spread of the squares: if they line up, the result is consistent.
- A narrow diamond close to 1 can be significant yet clinically small: translate it into an absolute effect.

## Common pitfalls

- Vote counting (2 positive and 2 negative, so we do not know): meta-analysis weighs and combines, it does not count.
- Reading the wrong side of the axis as favouring the intervention.
- Forgetting that the ratio axis is logarithmic: 0.5 and 2 are the same distance from 1.

## Memory aid

Squares are the trees, the diamond is the forest: look at the forest against the line at 1 without getting lost in one tree.

## Test yourself

Think of the answer, then tap the question to see it.

## What does the size of each study's square in a forest plot show?

The study's weight in the meta-analysis, which depends mainly on its precision (sample size and number of events).

## Where is the line of no effect for a relative risk and for a mean difference?

At 1 for the relative risk (and other ratios) and at 0 for a mean difference.

## The diamond of an RR forest plot runs from 0.65 to 0.94. What does it mean?

That the pooled effect is a statistically significant reduction: the whole CI lies below 1.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/epidemiology/forest-plot/)

## Connections

## Related 5

- [Meta-analysis](https://durgical.com/en/epidemiology/meta-analysis/)
- [Heterogeneity (I²)](https://durgical.com/en/epidemiology/heterogeneity/)
- [Confidence interval](https://durgical.com/en/epidemiology/confidence-interval/)
- [Relative risk](https://durgical.com/en/epidemiology/relative-risk/)
- [Systematic review](https://durgical.com/en/epidemiology/systematic-review/)

[← All of epidemiology](https://durgical.com/en/epidemiology/)

Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.

Sources: [Chapter 10: Analysing data and undertaking meta-analyses](https://www.cochrane.org/authors/handbooks-and-manuals/handbook/current/chapter-10) (Cochrane, 2026).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/epidemiology/forest-plot/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/epidemiology/forest-plot/