Comparing areas

On Twitter over the weekend I saw a couple of retweets of an infographic comparing the money raised to combat various diseases and the number of deaths caused by those diseases in the US. The infographic is interesting. It is also unintentionally misleading within each column.

Money raised vs deaths
An infographic comparing money raised for various diseases and the deaths for the same diseases. The original source is unknown. I have included the image here for critique.

The problem stems from the fact that 2D shapes (circles) have been used to represent the values but the infographic artist has scaled the shapes by their diameters. When we look at the graphic we naturally compare the areas of the circles. Unfortunately, areas scale as the square of the diameter: if a value is twice as big and you double the diameter, you've made the area four times as big. As a result, this infographic distorts the relative values. For instance it makes it look as though breast cancer accounts for about 72% of the money raised when it is actually about 51%. In the deaths column, it looks as though heart disease accounts for about 92% of deaths whereas it is actually 64%.

I've made a version of the infographic to show the difference between scaling the circles by diameter and scaling them by area.

Money raised vs deaths
A comparison of comparisons. The money raised and the deaths with the circles scaled by diameter (misleading) and by area.

The moral of this post: if you are using 2D shapes to show relative values, make sure you scale them correctly.

Posted in astro blog by Stuart on Sunday 24th Aug 2014 (23:35 BST) | Permalink
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