Posted on February 3, 2009

Racial Profiling Methods May Be Flawed

Philip Ball, Nature News, February 2, 2009

Singling out individuals for surveillance, investigation or screening on the basis of their race or nationality is probably useless for catching those engaged in or planning criminal acts. That is the conclusion of a computer scientist who has exposed the basic mathematical flaw in this approach to crime prevention.

At first glance, the approach seems logical, despite many people’s moral objections. If all previous acts of politically motivated terrorism have been committed by a particular nationality, then doesn’t it make sense to focus searches on those groups?

Not necessarily, says William Press of the University of Texas at Austin. Do the maths and you discover that a simple-minded application of these actuarial methods is worthless: all you end up doing is repeatedly picking out the same innocent people [1].

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Worse than chance?

It is hard to know how often actuarial selection is really used. Airport security policy, for example, is a closely guarded secret. {snip}

Some think that ethnicity is so clearly a key factor in the process of identifying potential criminals that it has become satirized as the new ‘crime’ of FWM, or Flying While Muslim–the latest variant of the US driving ‘offence’ called DWB: Driving While Black.

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Press says that, if ‘prior probabilities’ of an individual being a potential offender are assigned accurately, and if screening always results in accurate detection (neither of which is guaranteed), then the chance of finding those with true criminal intent is best if the search is conducted from the top down: from the highest to the lowest probability.

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One ‘obvious’ method is to make the chance of screening proportional to the chance of being an offender. But Press shows that the number of times this approach repeatedly picks out innocent individuals means that offenders are found no more efficiently than in a random search.

Thus, if actuarial screening is being done, says Press, the chances are that it’s being done ineffectively.

Counterproductive

This doesn’t mean that the technique has no benefit. Press calculates that the best strategy picks out high-risk individuals with a bias proportional to the square root of their probability.

That might be feasible if reliable, quantitative actuarial data are gathered in advance and selection is made by computer. But Press feels that, “given how socially fraught the issue is already, and given how weakly profiling helps, we shouldn’t do it at all”.

These calculations add weight to other arguments against actuarial methods. Harcourt [Bernard Harcourt, a professor of law and political science the University of Chicago], for instance, says that these approaches may be counterproductive in crime prevention because people are likely to change their behaviour in response. {snip} In fact, says Harcourt, these techniques “may actually increase crime, depending on how different populations respond to increased policing”.

Given Press’s result, Harcourt adds, “there is absolutely no reason left to profile.”

References

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