Stats for Part I offenses are based on incidents reported to the police, not arrests made by the police. These offenses include homicide, aggravated assault, robbery, burglary, rape, theft, etc. This is how cities typically measure their crime rate and how high crime areas are identified.

Stats for Part II offenses are based on arrests only. These offenses include minor assaults, fraud, forgery, drugs crimes, prostitution, weapon violations, loitering, etc. A large subset of Part II offenses do not have a specific victim so the stats are driven by police enforcement.