THE striking drop in crime over the last few years, particularly in New York City, but also notable in some other cities, undoubtedly has many causes.  The changes in policing methods have attracted the most attention.  Police departments—and this is particularly the case in New York—have discovered the efficacy of problem-oriented policing, developing methods of collecting data on crime rapidly and accurately, mapping the location of different kinds of crime, discovering concentrations of and connections among crimes, deploying resources strategically, and anticipating trends.  They have developed goals that measure not process but achievement—for example, not how many arrests have been made but has there actually been a reduction in crime.