but that's based on the assumption that you have a good understanding of the demographic makeup of local criminal gangs. which you don't, because criminals don't report their gang affiliation on their census form
and even if you did, to reasonably apply that information you need to have a good understanding of statistics. which you don't, because you're a police officer out on patrol who has to make quick decisions about whether to intervene in a potential crime in progress, and most people don't understand statistics even when they have a long time to spend thinking about it in a stress-free situation, and if you were REALLY that much of a statistics prodigy you would not be a cop because you would be using your skills to get rich on the stock market