Statistics How To

Forest Plot / Blobbogram: Definition, Simple Example

Descriptive Statistics > Forest Plot

What is a Forest Plot?


Image: James Grellier | Wikimedia Commons.

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:

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.

forest plot

Studies A and C show no difference, while B D and E show a significant difference.

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.

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

2 thoughts on “Forest Plot / Blobbogram: Definition, Simple Example

  1. Chris

    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 :)

  2. Andale Post author

    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!