How to read a forest plot

A forest plot is a graphical representation of a meta-analysis. It is usually accompanied by a table listing references (author and date) of the studies included in the meta-analysis. In the example here, there were three studies included in the meta-analysis: Evans 2011, Jones 2010 and Smith 2013. The table also lists the mean scores and standard deviations of these scores from each of the included studies; and it lists the number of participants in each study (under ‘Total’).

The forest plot is the graph on the right-hand side. It has one line representing each study in the meta-analysis, plotted according to the standardised mean difference (SMD – very roughly, this is the difference between the average score of participants in the intervention group, and the average score of participants in the control group). The green box on each line shows you the SMD for each study.

In the example, the first line represents the SMD of scores from the Evans 2011 study, (SMD=-0.50); the second line represents the SMD for scores from the Jones 2010 study (SMD=-0.30); and the third line represents these scores from the Smith 2013 study (SMD=-0.50).

The black diamond at the bottom of the graph shows the average effect size of the three studies.