Cartograms are hard

This cartogram, purporting to show the indebtedness of Eurozone countries, has been making the rounds on the internet. To me it mostly shows that making cartograms that can actually give the reader any relevent insights is hard, and should be left to specialists.

A cartogram is a map in which the size of the countries (or municipalities, states etc.) do not correspond to their geographical dimensions but instead are scaled according to a different variable, e.g. population. (Stricly speaking, a cartogram is not a map, but in layman’s terms it is.) Truly great cartograms exist, but in my experience, 99 per cent of the time cartograms muddle the data and don’t help gain insights.

This particular cartogram comes from a Deutsche Bank research report (p. 51). The report gives no source, nor any numbers in tabular form, nor even an explanation what the data being shown is. (“Sovereign debt” might mean a variety of things.) The different colors don’t seem to signify anything.

The worst feature of the map is that only Eurozone countries have been rescaled according to debt, whereas non-Eurozone countries (nearly half of the countries shown) retain their original size (but not shape, except for UK). Unless the reader knows all the 19 Eurozone countries by heart (and recognizes their distorted, unlabeled shapes on the map), she can only guess which countries’ sizes show relevant information and which do not.

Making the assumption that the data shown in the original cartogram is public sector liabilities minus assets, per capita (excluding social security funds for better comparability across countries) I downloaded the latest (Q1/2014) available data from Eurostat and created this simple horizontal bar chart.

Edit 23.3.: It would seem that this assumption is wrong. The data shown on the cartogram can’t be liabilities minus assets, whether or not social security funds are excluded, since the Eurostat numbers for those don’t match the relative sizes of the countries on the map. (For example, Belgium should make the top 2 in both cases, but is not in the top 10 of largest countries on the map.) Either the numbers are based on a different definition of sovereign debt, or are plain wrong. I’m guessing the latter, since I can’t imagine a definition of debt which would place Belgium very far from the top among European countries.

(I also collected the data into a single csv file for anyone wishing to create their own visualizations or analyses using the same data.)

Compare the bar chart and cartogram and decide for yourself which gives you more insights about the underlying data. I think the answer is pretty obvious.


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