The Stop and Frisk sleight of hand
I’m finishing up an essay called “On Being a Data Skeptic” in which I catalog different standard mistakes people make with data – sometimes unintentionally, sometimes intentionally.
It occurred to me, as I wrote it, and as I read the various press conferences with departing mayor Bloomberg and Police Commissioner Raymond Kelly when they addressed the Stop and Frisk policy, that they are guilty of making one of these standard mistakes. Namely, they use a sleight of hand with respect to the evaluation metric of the policy.
Recall that an evaluation metric for a model is the way you decide whether the model works. So if you’re predicting whether someone would like a movie, you should go back and check whether your recommendations were good, and revise your model if not. It’s a crucial part of the model, and a poor choice for it can have dire consequences – you could end up optimizing to the wrong thing.
[Aside: as I’ve complained about before, the Value Added Model for teachers doesn’t have an evaluation method of record, which is a very bad sign indeed about the model. And that’s a Bloomberg brainchild as well.]
So what am I talking about?
Here’s the model: stopping and frisking suspicious-looking people in high-crime areas will improve the safety and well-being of the city as a whole.
Here’s Bloomberg/Kelly’s evaluation method: the death rate by murder has gone down in New York during the policy. However, that rate is highly variable and depends just as much on whether there’s a crack epidemic going on as anything else. Or maybe it’s improved medical care. Truth is people don’t really know. In any case ascribing credit for the plunging death rate to Stop and Frisk is a tenuous causal argument. Plus since Stop and Frisk events have decreased drastically recently, we haven’t seen the murder rate shoot up.
Here’s another possible evaluation method: trust in the police. And considering that 400,000 innocent black and Latino New Yorkers were stopped last year under this policy (here are more stats), versus less than 50,000 whites, and most of them were young men, it stands to reason that the average young minority male feels less trust towards police than the average young white male. In fact, this is an amazing statistic put together by the NYCLU from 2011:
The number of stops of young black men exceeded the entire city population of young black men (168,126 as compared to 158,406).
If I’m a black guy I have an expectation of getting stopped and frisked at least once per year. How does that make me trust cops?
Let’s choose an evaluation method closer to what we can actually control, and let’s optimize to it.
Update: a guest columnist fills in for David Brooks, hopefully not for the last time, and gives us his take on Kelly, Obama, and racial profiling.