How to Create a Forest Plot in R


A forest plot (sometimes called a “blobbogram”) is used in a meta-analysis to visualize the results of several studies in one plot.

The x-axis displays the value of interest in the studies (often an odds ratio, effect size, or mean difference) and the y-axis displays the results from each individual study.

This type of plot offers a convenient way to visualize the results of several studies all at once.

The following example shows how to create a forest plot in R.

Example: Forest Plot in R

To create a forest plot in R, we need to first create a data frame to hold the effect size (or whatever value of interest) and the upper and lower confidence intervals for each study:

#create data
df <- data.frame(study=c('S1', 'S2', 'S3', 'S4', 'S5', 'S6', 'S7'),
                 index=1:7,
                 effect=c(-.4, -.25, -.1, .1, .15, .2, .3),
                 lower=c(-.43, -.29, -.17, -.02, .04, .17, .27),
                 upper=c(-.37, -.21, -.03, .22, .24, .23, .33))

#view data
head(df)

  study index effect lower upper
1    S1     1  -0.40 -0.43 -0.37
2    S2     2  -0.25 -0.29 -0.21
3    S3     3  -0.10 -0.17 -0.03
4    S4     4   0.10 -0.02  0.22
5    S5     5   0.15  0.04  0.24
6    S6     6   0.20  0.17  0.23
7    S7     7   0.30  0.27  0.33

Next, we can use functions from the ggplot2 data visualization package to create the following forest plot:

#load ggplot2
library(ggplot2)

#create forest plot
ggplot(data=df, aes(y=index, x=effect, xmin=lower, xmax=upper)) +
  geom_point() + 
  geom_errorbarh(height=.1) +
  scale_y_continuous(name = "", breaks=1:nrow(df), labels=df$study)

The x-axis displays the effect size for each study and the y-axis displays the name of each study.

The points in the plot displays the effect size for each study and the error bars show the confidence interval bounds.

Note that we can also add a title, modify the axis labels, and add a vertical line at an effect size of zero to make the chart more aesthetically pleasing:

#load ggplot2
library(ggplot2)

#create forest plot
ggplot(data=df, aes(y=index, x=effect, xmin=lower, xmax=upper)) +
  geom_point() + 
  geom_errorbarh(height=.1) +
  scale_y_continuous(breaks=1:nrow(df), labels=df$study) +
  labs(title='Effect Size by Study', x='Effect Size', y = 'Study') +
  geom_vline(xintercept=0, color='black', linetype='dashed', alpha=.5) +
  theme_minimal()

Forest plot in R

Feel free to modify the theme of the plot to make it look however you’d like. For example, we could also use theme_classic() for an even more classic appearance:

#load ggplot2
library(ggplot2)

#create forest plot
ggplot(data=df, aes(y=index, x=effect, xmin=lower, xmax=upper)) +
  geom_point() + 
  geom_errorbarh(height=.1) +
  scale_y_continuous(breaks=1:nrow(df), labels=df$study) +
  labs(title='Effect Size by Study', x='Effect Size', y = 'Study') +
  geom_vline(xintercept=0, color='black', linetype='dashed', alpha=.5) +
  theme_classic()

Additional Resources

How to Create a Forest Plot in Excel
A Complete Guide to the Best ggplot2 Themes

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