Thursday, August 28, 2008

The Atlantic Monthly is criminally misusing statistics

I spent the last week vacationing in South Carolina, where my parent's house seems to have Atlantic Monthly's and Harper's from the dawn of time. What luck, then, that one of the most interesting articles (at least statistically) was in an issue as recent as the July/August 2008 issue of the Atlantic. The article is called "American Murder Mystery" and it's by Hanna Rosin.

The article talks of the recent increase in violent crime in mid-sized cities. In many of these cities, government housing projects (called "Section 8" housing) have been torn down. In their place, the government has provided the poor with rent subsidies so that they can move to private housing. Rosin describes how Phyllis Betts and Richard Janikowski, of the University of Memphis, tie the increase in crime in these cities to the destruction of these projects. A striking quote in the article is from the Memphis police chief: '“It used to be the criminal element was more confined,” said Larry Godwin, the police chief. “Now it’s all spread out."'

The primary statistical evidence given in the article of an association between crime and former Section 8 residents, is a map that shows areas with high incidents of crime correspond to areas with a large number of people with Section 8 subsidies (i.e., former residents of housing projects). As convincing as this might sound, it has a fatal flaw: the map looks at total incidents rather than crime rate. This means that an area with 10,000 people and 100 crimes (and 100 Section 8 subsidy recipients) will look much worse than an area with 100 people and 1 crime (and 1 Section 8 subsidy recipient). However, both areas have the same rate of crime, and, presumably, the same odds of being a victim of crime (see my earlier blog about the safest place to live for some explanation of the use of rates in measuring crime). Yet in Betts and Janikowski's analysis, the area with 10,000 people has a higher number Section 8 subsidy recipients and higher crime, thus "proving" their theory of association.

Of course, there will be both a greater number of Section 8 subsidy recipients and a greater number of crimes in the area with 10,000 people than in the area with 100 people . Thus, while the map presented in the Atlantic article does indeed seem to indicate that there is higher crime in areas where there are more Section 8 subsidies, this differential might be entirely an artifact of population density, and, in fact, the crime rate may be completely unrelated to where Section 8 subsidy recipients reside. Without an adjustment for population density, the inferences made from the association are statistically meaningless.