Yesterday Ray Kelly took to the pages of the Wall Street Journal to defend NYPD's Stop and Frisk tactics and its indiscriminate spying on Muslim communities:

Since 2002, the New York Police Department has taken tens of thousands of weapons off the street through proactive policing strategies. The effect this has had on the murder rate is staggering. In the 11 years before Mayor Michael Bloomberg took office, there were 13,212 murders in New York City. During the 11 years of his administration, there have been 5,849. That's 7,383 lives saved–and if history is a guide, they are largely the lives of young men of color. So far this year, murders are down 29% from the 50-year low achieved in 2012, and we've seen the fewest shootings in two decades.

To critics, none of this seems to much matter. Sidestepping the fact that these policies work, they continue to allege that massive numbers of minorities are stopped and questioned by police for no reason other than their race.

As one of Ray Kelly's critics, and a citizen of New York, I will say that the declining murder rate matters a great deal. But the question before us isn't "Do we want the murder rate reduced?" The question is "Is Stop and Frisk a moral and effective policy?" We could also start punishing all murderers with public torture and beheading. That too might reduce the murder rate. Or perhaps the murder rate might fall for less conspicuous reasons, and those who endorsed public beheadings can loudly claim the credit anyway. At least we'd have correlation. Presently that is more than you can say for Stop and Frisk. Kelly rightly points out that the murder rate in our great city is falling. But for some reason he neglects to mention that Stop and Frisk numbers are falling too.

Perhaps there is some relationship between the long drop in homicides and Stop and Frisk, but Ray Kelly has never furnished such actual proof. Understanding why crime rises and falls has bedeviled social scientists for decades, so it's not surprising that Kelly would have trouble offering hard evidence. But we can certainly examine Ray Kelly's claim that Stop and Frisk is responsible for large numbers of weapons coming off the street.