I don't know if this applies much anymore, but they used talk about how political the crime stats are. If a mayor who campaigned as though on crime wants to look good, there are ways to cook the books, so to speak. Eg., if the sizzlean bust a barroom brawl, they used to do things like report 1 criminal incident rather than the likely multitudes of crimes they actually might count. If I'm remembering right, they might report it as one incident when, in fact, there were three people charged. The reverse used to happen as well: cops would inflate the books to make crime look worse. ISTR, though, that there was reforms handed down after the above research revealed how political the stats were. But IASTR that there are still huge differences between cities and how they choose to count crime: Uniform Crime Report in name only.
NYC's get tough on crime mayors can still cook the books at a different level. Cops can be urged to harass folks while simply not apprehending or booking. That way, Ghouliani and his successor could demonstrate that the tough on crime stance worked Because, at that level, there still is a great deal of politicization as to whether people are actually booked or not. Or, get tough on crime and book 'em danno. Show how the increased numbers of sizzlean worked and then change policies to reveal--presto!--lower crime rates.
I recall in the old 'hood how the a cop actually first visited the person reporting a potential crime before they actually trying to apprehend the criminal. cute, huh? having dealt with them, it was clear they were hoping for two crimes. after all, the person reporting in that neighborhood could probably ALSO be busted on something too cause we're poor and thus guilty of something!
But, also, see this: http://www.bja.evaluationwebsite.org/html/documents/measures_of_effectiveness.htm
"We're in a fucking stagmire."
--Little Carmine, 'The Sopranos'