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Few systematic reviews containing meta-analyses are complete without a forest plot. But what are forest plots, and where did they come from?
Summary points
Forest plots show the information from the individual studies that went into the meta-analysis at a glance
They show the amount of variation between the studies and an estimate of the overall result
Forest plots, in various forms, have been published for about 20 years
During this time, they have been improved, but it is still not easy to draw them in most standard computer packages
What is a forest plot?
In a typical forest plot, the results of component studies are shown as squares centred on the point estimate of the result of each study. A horizontal line runs through the square to show its confidence interval-usually, but not always, a 95% confidence interval. The overall estimate from the meta-analysis and its confidence interval are put at the bottom, represented as a diamond. The centre of the diamond represents the pooled point estimate, and its horizontal tips represent the confidence interval. Significance is achieved at the set level if the diamond is clear of the line of no effect.
The plot allows readers to see the information from the individual studies that went into the meta-analysis at a glance. It provides a simple visual representation of the amount of variation between the results of the studies, as well as an estimate of the overall result of all the studies together. Forest plots increasingly feature in medical journals, and the...