Compstat, a statistical system for tracking crime, was introduced by Police Commissioner Bill Bratton and his deputy, Jack Maple, in 1994, during Bratton’s first stint as head of the NYPD. Twenty-two years later, Bratton is once again the city’s top cop, Compstat is still in use, and crime has fallen 75 percent. New York had 1946 murders in 1993, or...