Testing for Racial Discrimination in Police Searches of Motor Vehicles

Source: Camelia Simoiu, Sam Corbett-Davies, Sharad Goel, Stanford University, 2016

In the course of conducting traffic stops, officers have discretion to search motorists for drugs, weapons, and other contraband. There is concern that these search decisions are prone to racial bias, but it has proven difficult to rigorously assess claims of discrimination. Here we develop a new statistical method —the threshold test— to test for racial discrimination in motor vehicle searches. We use geographic variation in stop outcomes to infer the effective race-specific standards of evidence that officers apply when deciding whom to search, an approach we formalize with a hierarchical Bayesian latent variable model. This technique mitigates the problems of omitted variables and infra-marginality associated with benchmark and outcome tests for discrimination. On a dataset of 4.5 million police stops in North Carolina, we find that the standard for searching black and Hispanic drivers is considerably lower than the standard for searching white and Asian drivers, a pattern that holds consistently across the 100 largest police departments in the state.
Stanford researchers develop new statistical test that shows racial profiling in police traffic stops
Source: Edmund Andrews, Stanford University, Press Release, June 28, 2016

The new tool shows that police in North Carolina were more likely to search black and Hispanic motorists than white ones.

How can we improve the criminal justice system?
Source: Edmund L. Andrews, Stanford University, Engineering, Press Release, February 10, 2016

Engineers use computational analysis tools to reveal discrimination and reduce crime.

Test suggests North Carolina police target black drivers
Source: Tom Abate, Futurity, June 29, 2016

Police in North Carolina are more likely to search black and Hispanic motorists than white or Asian drivers, according to a new study of 4.5 million traffic stops.

But while blacks and Hispanics are more likely to be searched, those searches are less likely to uncover illegal drugs or weapons than searches of vehicles with white or Asian drivers.

Studies based on the incidence of searches by race, and the outcomes of those searches, have been done in the past, but for the new study, researchers developed a third, entirely new measurement—a threshold test.

The researchers show that this new measure offers a statistically rigorous way to quantify how suspicious officers were to initiate a search. For example, did officers conduct searches when there was a 15 percent probability of finding weapons or drugs, or was a 5 percent inkling enough? They correlated these threshold assessments to the race or ethnicity of the subjects across the entire dataset of 4.5 million motor vehicle stops…..

8 ways police can cut bias against African Americans
Source: Clifton B. Parker, Futurity, June 17, 2016

New research on thousands of police interactions finds significant racial differences in how police in Oakland, California, treat African Americans during traffic and pedestrian stops.

The researchers suggest 50 measures, divided here into 8 broad types, to improve police-community relations in Oakland and elsewhere, such as better data collection, bias training, and changing cultures and systems.