“Stop and frisk” statistics

by Andrew Gelman on July 17, 2013 · 2 comments

in Race,Violence

Washington Post columnist Richard Cohen brings up one of my research topics:

In New York City, blacks make up a quarter of the population, yet they represent 78 percent of all shooting suspects — almost all of them young men. We know them from the nightly news.

Those statistics represent the justification for New York City’s controversial stop-and-frisk program, which amounts to racial profiling writ large. After all, if young black males are your shooters, then it ought to be young black males whom the police stop and frisk.

I have two comments on this. First, my research with Jeff Fagan and Alex Kiss (based on data from the late 1990s, so maybe things have changed) found that the NYPD was stopping blacks and hispanics at a rate higher than their previous arrest rates:

To briefly summarize our findings, blacks and Hispanics represented 51% and 33% of the stops while representing only 26% and 24% of the New York City population. Compared with the number of arrests of each group in the previous year (used as a proxy for the rate of criminal behavior), blacks were stopped 23% more often than whites and Hispanics were stopped 39% more often than whites. Controlling for precinct actually increased these discrepancies, with minorities between 1.5 and 2.5 times as often as whites (compared with the groups’ previous arrest rates in the precincts where they were stopped) for the most common categories of stops (violent crimes and drug crimes), with smaller differences for property and drug crimes.

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I can’t fault Cohen here, he’s just a newspaper columnist, you can’t expect him to be aware of a six-year-old article in the Journal of the American Statistical Association. And things may have changed since 1998-1999 (which is when our data are from). But the data we have here shows the police were disproportionately stopping minorities.

The other thing is, I don’t think Cohen is necessarily being fair to the police when he describes the stop-and-frisk program as “racial profiling.” As we wrote in our paper, “It is quite reasonable to suppose that effective policing requires stopping and questioning many people to gather information about any given crime.” It could well be that a statistical pattern of stops could arise from individual decisions that are not based on race but instead are based on characteristics that are correlated with race. I have no idea what the police are doing—-my only experience here is with the numbers.

I that Cohen is, on one hand, way too quick to dismiss the numbers with his blanket statement that “young black males are your shooters” and on the other hand may be way too quick to describe police work as racial profiling.

P.S. As a bonus, Slate columnist Matthew Yglesias connects this to one of my other research interests: Bayesian inference. I won’t comment on Yglesias’s remarks except to point out that Bayes’ theorem is a two-way street. The problem here is not so much Bayesian inference as its application to decision analysis. If you have to make individual decisions by maximizing the probability of success (catching a criminal, if you are the police), then profiling can be a logical strategy. Reasons not to profile include, “equal protection of the laws” etc. and also indirect effects of what one might call the “profiling culture,” effects such as hassling innocent people, reducing trust in the police, empowerment of Bernard Goetz and George Zimmerman to go around shooting people, etc. Bayes’ theorem is relevant in all these calculations but the issues here are not trivial.


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