The Mathematics of the College Graduate Rate Map

Part of the discussion about cities that attract the creative class has revolved around a set of two thematic maps that show the distribution of college graduates in US counties in 1970 and 2000. In 1970, 11% of the US population had a bachelor’s; in 2000, 24% did. Both maps are divided into five colors – off-white for counties lower than the national average by more than 10 percentage points, yellow for counties lower by 4-10, ochre for counties within 4 percentage points of the national average, burnt sienna for counties higher by 4-10, and red for counties higher than 10.

The 1970 map is dominated by ochre in the Northeast and West and yellow in the Midwest and South; the 2004 map is dominated by off-white, with patches of all darker colors scattered around, especially in New England. Amanda said that it’s evidence that college graduates are clustering in college towns. Gordo says it’s just evidence that there are more college graduates in the US now.

In fact, most of the clustering can’t be explained just by the increase in college graduation rates. Plenty of counties switched from ochre to off-white between 1970 and 2000; in other words, between 7 and 15 percent of their population had a college degree in 1970, and less than 14% did in 2000. This suggests that these counties, which constitute a substantial chunk of the Interior West, Texas, South Carolina, and Pennsylvania, have apparently stagnated.

Just the existence of off-white counties in 2000 where there were none in 1970 isn’t evidence of increased clustering of educated Americans. However, the fact that many counties switched from ochre to off-white is.

In addition, there’s the cluster pattern of the 2000 map. If the only difference between 1970 and 2000 were an increased number of college graduates, the two maps would have similar clustering patterns, only the 2000 one would have a starker color contrast. But in fact, 2000 really is more clustered; there are burnt sienna and red counties that used to be ochre.

Incidentally, one feature that nobody seems to have paid attention to is the nature of the clustering. What little clustering existed in 1970 was based in specific cities, while by 2000 it spread to neighboring counties. This is especially true in the Minneapolis, Atlanta, and DC areas, which increased from 1, 1, and 4 burnt sienna or red counties respectively to 6, 6, and 10. That indicates that the growth of education was not just in the cities but also in the suburbs. I’d say it connects to the liberalization of the Northern suburbs in the US, but this is also apparent in the Atlanta suburbs; Cobb County, which issued stickers in biology textbooks saying evolution was just a theory, is red, while New York City’s outer boroughs are all yellow except Bronx, which is off-white.

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