The choropleth problem

Before a reader draws me up short on Monday's link to an interactive map showing explosive growth of unemployment in the US, I should acknowledge the choropleth problem. James Fallows introduced the issue, and the word, in a blog post about the same map Tuesday. The problem is that geography does not equal population. A choropleth map depicting social trends (unemployment or election results) can mislead if its geographical units (states or provinces) vary widely in population. (The word derives from Greek terms for "area/region" + "multiply.") Fallows gives the example of the razor thin 2004 US presidential election, in which the...