Dark Figure Crime
The UCR only reflects data that was reported to them. This data is made up of crimes reported to police agencies, and does not include unreported, or "dark figure," crime.
The UCR only reflects crimes that directly hurt a person or property. Crimes such as prostitution are considered victimless crimes and are not reported.
Rapes reported to the UCR only include females who've been rapped, not male or same-sex rape.
Crimes that involve multiple-offense crimes are only recorded as the most severe type. For example, a murder would outweigh arson and thus a murder/arson would only be reported as a murder.
It is not mandatory for an agency to report anything to the UCR. All reports to the UCR are strictly voluntary.
- police car up close image by Aaron Kohr from Fotolia.com