Clinical research & EBM · Reference
What is a forest plot?
A forest plot is a graph that displays the effect estimate and confidence interval from each study in a meta-analysis, alongside the pooled result. Each study is a square with a horizontal line; the combined estimate is the diamond at the foot of the plot.
Anatomy of the plot
Each row of a forest plot represents one study. A marker — usually a square — sits at that study’s effect estimate, such as a hazard ratio or relative risk, and a horizontal line through it shows the confidence interval. The size of the square reflects the study’s weight in the analysis, so larger, more precise studies draw the eye. A vertical line of no effect runs down the plot (at 1 for ratio measures, 0 for differences); a confidence interval that crosses it indicates a result not statistically significant at the chosen level.
The diamond and the pooled estimate
At the foot of the plot sits the diamond, which represents the pooled estimate combining all the studies. Its centre marks the combined effect and its width shows the confidence interval of that pooled result. Because the diamond aggregates many studies, it is usually narrower — more precise — than any single study’s line. Where the diamond falls relative to the line of no effect summarises the meta-analysis: clearly to one side suggests a consistent effect, while a diamond straddling the line suggests the combined evidence is inconclusive.
Reading heterogeneity
A forest plot also reveals heterogeneity — how much the studies disagree. If the study markers line up and their intervals overlap, the effect looks consistent; if they scatter widely, the studies may be measuring different things or differ in design and population. This visual impression is usually accompanied by statistics such as I², which quantifies the proportion of variation due to genuine differences rather than chance. Forest plots are a standard feature of systematic reviews and are produced to the reporting standards set out by PRISMA.
Key facts
At a glance
- Definition: Graph of study estimates in a meta-analysis
- Each study: A square marker with a confidence-interval line
- Square size: Reflects the study’s weight in the analysis
- The diamond: The pooled estimate combining all studies
- Line of no effect: At 1 (ratios) or 0 (differences)
- Also shows: Heterogeneity between studies
Common questions
FAQ
What does the diamond on a forest plot mean?+
The diamond represents the pooled estimate that combines all the studies in the meta-analysis. Its centre marks the combined effect and its width is the confidence interval of that pooled result, which is usually narrower than any single study because it aggregates the evidence.
What does it mean if a study’s line crosses the line of no effect?+
The vertical line of no effect sits at 1 for ratio measures or 0 for differences. If a study’s confidence-interval line crosses it, the result is not statistically significant at the chosen level, because the interval includes the value that means no difference between groups.
How does a forest plot show heterogeneity?+
When the study markers line up and their confidence intervals overlap, the effect looks consistent; when they scatter widely, the studies disagree. This visual impression is usually paired with a statistic such as I², which quantifies how much of the variation reflects genuine differences rather than chance.
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