Descriptive Statistics > Forest Plot

## What is a Forest Plot?

A forest plot (sometimes called a blobbogram) is a graph that

**compares several clinical studies.**The graph has several components:

- A
**vertical line**in the center. This is the line of equality, or no effect. If the blobbogram is a relative risk ratio, the vertical line will have no effect at zero; if it shows a mean difference, the null is 0. For ratios (e.g. the odds ratio) the line is at 1. - A
**horizontal line**representing each study. Sometimes this will be a horizontal bar. The width of the bar represents the confidence interval, usually the 95% interval. This is the range where the true value is likely to fall. The**diamond/point/square**in the center of the line is a point estimate of the true value. The bigger the shape, the larger the sample size. This is the*most likely*value out of the range of possible values; values towards the end of the line are less likely. - An
**outline of a diamond**at the base of the graph. This usually represents a weighted average for all studies but it can also be an odds ratio. This should be clearly labeled as either statistic. In the image above, it’s an odds ratio. - An adjacent
**table**provides more information about the study. The table can include:- Study author and date.
- Mean scores and standard deviations.
- Total number of participants in each study.

## 1. A Simple Forest Plot Example

The horizontal line is perhaps the most important part of the graph. When a line representing a study crosses the vertical line, it represents no difference. If all of the horizontal lines cross the vertical line, it’s a sign that all of the studies were in agreement. What’s really of interest is if a horizontal line *doesn’t *cross the vertical — that’s an indication there were statistically significant differences between studies.

The line is a representation of mean difference (a statistic that measures the absolute value of two results) or standardized mean difference. This is sometimes called a “weighted mean difference,” but this is a bit of a misnomer as no weighting is actually involved in the calculations. The length of the line indicates the

**confidence interval**: longer lines mean more uncertainty. Optional tick marks represent the 95% and 99% confidence intervals. Comments are now closed for this post. Need help or want to post a correction? Please post a comment on our Facebook page and I'll do my best to help!

Forest Plot / Blobbogram: Definition, Simple Example was last modified: October 12th, 2017 by

Hi, your website is amazing! Shouldn’t the line of no effect on a forest plot bisect the x-axis at 1 when the data is presented in terms of odds ratios? Above it says zero.

Thanks :)

Yes, the line of no effect should be at 1. I added a note to that effect, I hope it makes the meaning clearer :)

And thanks…glad you’re finding the site useful!