Mr. Zakaria engages in over-reach with respect to the Harvard-Berkeley study. He is not the first. The Harvard-Berkeley study was not about urban form, cities or segregation. It was about the relationship between income mobility and tax expenditures.
The authors provided additional information with 25 separate, simple correlation analyses between 25 individual variables and economic mobility (demographic factors were not controlled). Co-Author Raj Chetty described this supplemental research in a PBS interview, citing income segregation, school quality, two-parent families and measures, civic engagement, religiosity and community cohesiveness. The authors urged caution in interpreting these correlations: “For instance, areas with high rates of segregation may also have other differences that could be the root cause driving the differences in children’s outcomes.”
Columnists and some analysts have taken simple correlation analyses
— which does not control for other factors (not even the other 24 that were calculated) — to make their urban form points. In contrast, perhaps the most rational voice was Columbia University urban planning professor David King, who noted that: “…snapshot correlations really don’t mean anything and will provide evidence for whatever point of view is desired.”
Simple correlation analysis is not enough. There is a need to control for other important factors. That’s why we have econometric analysis.
This issue is discussed in greater detail in “Distortions and Reality about Income Mobility” at http://goo.gl/xSM6v7