Area vs radius

By Aaron Schiff 04/04/2015


Stats NZ have just published* the 2014 regional GDP statistics, together with an attractive infographic that shows how national GDP is distributed across the regions.

The map part of the infographic caught my eye:

statsgraphic

A crucial issue with this sort of picture is whether to use the area or the radius of the circles to represent the data. In this case it looks like the area has been used. There is a good argument that areas are difficult to compare, and so radius is the better measure.

This is an easy trap to fall into and I fell into it myself just the other day so I feel a bit bad about being critical of Stats here. But this is important because it can greatly affect the perception of the share of economic activity that occurs in different regions. To illustrate, I’ve made two maps, the one on the left uses area of the circles to represent regional GDP (the picture looks similar to Stats NZ’s one) and the one on the right uses radius. In both cases I’ve scaled so that the lowest GDP regions (Gisborne and West Coast) are just visible (so you can’t compare across the two pictures, they are on different scales).

gdpcomparison

Quite a difference! In this case, a plain old bar chart might be better, which Stats also provide in their infographic:

statsbar

* Editor’s note: we continue to work through Aaron’s excellent back-catalogue of posts.