Population vs. Per Capita

Sometimes at work we get requests to visualize data on maps.  It’s a really cool feature, but the challenge is generally the calculation being used to drive the chart is a straight sum, and states like California, New York, and Texas always seem to have the highest values.

XKCD had a great comic the other day that I 100% agree with: the problem with geographic heat maps is that it’s essentially just a population map.

How clever of you to see if there would be alt-text.


This is why anytime you’re putting together a heat map, it’s best to normalize the data as best as you can with a per-capita calculation.

Compare the following two calculations:

  • Summation -> Sum(Measure)
  • Per Capita -> Sum(Measure)/Sum(Number of People in that Grouping)

This very simple switch allows you to make a much more effective comparison of large states like California to small states like Rhode Island.

USA Today offers a graph every morning in the bottom-left corner of the front page.

Take a look at this first one:

Why am I not surprised by these results?  It’s essentially a “top 5 population” map.  The only thing that seems off is that Arizona and Georgia are showing up here, but I’m bringing outside knowledge that the population is not very high there, so I can assume that it must be an outlier.

However a few weeks later I picked up the paper and pleasantly surprised to see this map:

Much better!  Now that they’ve switched to a percentage-focused view, I get a much better sense that in these states the proportions are indeed larger when compared to other states.

Map visualizations can be very powerful, but a little simple division can help you get a greater wealth of information from the same pixel space!

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