The Forest Plot and the Box-and-whisker Plot

Created on Wed, 04/19/2017 - 03:52
Last updated on Wed, 04/19/2017 - 03:52

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To quote the college, "candidates either knew this topic or knew nothing about it". We have all seen these graphs before, but when pushed to give specific definitions people tend to do poorly. Fortunately, there is not much to know.

The Forest plot has also been asked about in the Fellowship Exam ( Question 8 from the first paper of 2015 and Question 10 from the first paper of 2009), where the image below is borrowed. 

Anatomy of the Forest Plot

PPI vs H2A bleeding

  • The vertical line: line of "no effect", OR=1.0
  • The horizontal lines: the confidence interval of the individual study
  • The position of the square: a point estimate of the odds ratio (OR)
  • The size of the square: the weight of the study according to the weighing rules of the meta-analysis, likely representing the sample size and statistical power. 
  • The diamond at the bottom: the combined result of the trial
  • Results can be considered statistically significant if the confidence intervals of the combined result do not cross the line of no effect

Anatomy of the box and whisker plot

what is a boxplot.

The box and whisker plot is a way of graphically representing the "five number summary", or four parameters which demonstrate the central tendency of the data set.

  • Range (the minimum and maximum values)
  • First quartile (25%)
  • Third quartile (75%)
  • Median, represented by the vertical bar in the centre

Uses of this graph:

  • Good for comparing distributions (the centre, spread and overall range are immediately apparent)
  • Useful for indicating whether a distribution is skewed 
  • Detects outliers
  • Useful for summarising a set of data measured on an interval scale