Color scale

This map from the Economist illustrates pretty well the movement of population from middle America outwards from 2000-6.  The message reaches us despite the large volume of data painted.  (The gray shadow though was more than a little distracting.)

The map piqued my curiosity in two areas:

How did they determine the color scale?  The average change over all counties (6.4%) was obviously used.  Standard deviation was not since the ranges of change were unequal in size.

Was within-county percent change the best criterion?  As is, an 80% drop in a 2,000-people county looks the same as an 80% drop in a 200,000-strong county.

Reference: "The Great Plains drain", Economist, Jan 17 2008.

PS. I am traveling and so posting will be limited.

Comments

[Jim]

A map very similar to this and many others besides are published in the Census Atlas of the United States, worth a look for some beautiful examples of design:
http://www.census.gov/population/www/cen2000/censusatlas/

[John S.]

I agree that the Census Bureau produces some beautiful graphics. On the other hand, what do you make of the following:

http://www.census.gov/popest/gallery/maps/CBSA_perchg_2000_2006.html

It shows something about population change, but it's difficult to interpret. I guess darker colored regions have higher population growth than lighter colored regions, but the use of different colors for metropolitan and micropolitan areas makes for a confusing mess.