An overwhelming 84 percent of the stops in the first three-quarters of 2009 were of black or Hispanic New Yorkers. It is incredible how few of the stops yielded any law enforcement benefit. Contraband, which usually means drugs, was found in only 1.6 percent of the stops of black New Yorkers. For Hispanics, it was just 1.5 percent. For whites, who are stopped far less frequently, contraband was found 2.2 percent of the time.In this analysis lies a fundamental mistake. We have not been provided with the correct statistic to determine whether the department is actually racist. Suppose, as in the figure below (click to enlarge), that minority members of the population and majority members of the population factually have a different distribution of "suspiciousness."
For example, the x-axis could represent the level of "suspicious dress" (let's suppose that this is not endogenous). The top chart represents the distribution of the majority population, while the bottom chart represents the distribution of the minority population. In this example, there is little signaling differentiation among the minority population. However, there is a great deal among the majority population. In this sense, although the population averages are the same, and although the "level of suspiciousness" trigger is the same, a much larger proportion of the minority population appears suspicious.
If suspiciousness is correlated with commission of crimes, the New York Police would not be discriminating by stopping more black men. Moreover, if suspiciousness in the majority population is so rare that it strongly signals criminal behavior, we would expect a larger proportion of police stops of majorities to lead to arrest.
Ultimately, what determines discrimination is not the average number of police stops that lead to arrest of minorities vs. majorities. The proportion of marginal stops that lead to arrest , which we do not observe, determine the level of discrimination on the police force. If the marginal (last) stop of a minority is less likely to lead to arrest than the marginal (last) stop of a majority, then the police department would be discriminating by stopping the minority. The only way the statistics we are given could actually lead us to the conclusion that the New York Police Department is racist is if they were the results of marginal, not average stops. This is not the case; the Times is mistaken.