Education And Debate

Forest plots: trying to see the wood and the trees

BMJ 2001; 322 doi: http://dx.doi.org/10.1136/bmj.322.7300.1479 (Published 16 June 2001) Cite this as: BMJ 2001;322:1479
  1. Steff Lewis, medical statistician (steff.lewis@ed.ac.uk)a,
  2. Mike Clarke, associate director (research)b
  1. a Neurosciences Trials Unit, Department of Clinical Neurosciences, University of Edinburgh, Western General Hospital, Edinburgh EH4 2XU
  2. b UK Cochrane Centre, NHS Research and Development Programme, Oxford OX2 7LG
  1. Correspondence to: S Lewis
  • Accepted 20 February 2001

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 …

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