"'Crime in the United States' (CIUS), published annually by the FBI, is a compilation of the Uniform Crime Reports (UCR) provided by over 18,000 policing jurisdictions. It represents one of the two primary sources of data about crime in the United States, the National Crime Victimization Survey (NCVS) being the other The quality of the data provided to the FBI, however, is uneven. Reporting to the FBI remains for many jurisdictions a voluntary activity; although many States now mandate that agencies report crime and arrest data to them (which they then forward to the FBI), even in those States local agencies do not always comply. Moreover, despite the efforts of the FBI to maintain their quality, there are many gaps in the data that make their use questionable. While this has had limited impact in the past, the fact that the UCR data have, for the first time, been used to allocate Federal funds brings issues about data quality to center stage This report describes the history of the UCR system and the data problems that it deals with in reporting crime, arrest and homicide. It describes the procedures used by the FBI to fill in gaps in the data when they exist and makes suggestions about how they might be improved."
Office of Justice Programs, Bureau of Justice Statistics: http://www.ojp.usdoj.gov/bjs/