For almost 80 years the Uniform Crime Reports (UCR) have provided statistical data about violent and victim crimes. Although the UCR is the standard in criminal statistics, several fundamental flaws have been identified with the system.
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.