NYPD Stop and Frisks

Lenny Levitt poses an interesting question is his weekly column:

From 2004 through 2009, [New York City] police have had nearly three million stop-and-frisk encounters, which involve patting people down or questioning them. Virtually all of those stopped are black or Hispanic. In 88 per cent of the cases, the people searched or questioned were innocent of wrongdoing. [ed note: I think innocent is too strong a word for not arrested or given a citation. But regardless, many if not most of those frisked turn out to be guilty of nothing more than living in a neighborhood where police stop and frisk a lot people.]
Stop-and-frisk had been the tactic of the department’s former plainclothes Street Crime Unit, which prided itself on getting guns off the streets. After four improperly trained Street Crime cops shot and killed the unarmed Amadou Diallo in 1999, Police Commissioner Howard Safir put the unit into uniform, in effect destroying it. Upon taking office in 2002, Kelly abolished it entirely.

For the record, nobody wants a return to the random violence of the early 1990s. But is the current crime decline, which has continued for the past 15 years, in any way related to Kelly’s aggressive stop-and-frisk policies, which began in 2004?