Information on UCR from the FBI website
To ensure these data are uniformly reported, the FBI provides contributing law enforcement agencies with a handbook that explains how to classify and score offenses and provides uniform crime offense definitions. Acknowledging that offense definitions may vary from state to state, the FBI cautions agencies to report offenses not according to local or state statutes but according to those guidelines provided in the handbook. Most agencies make a good faith effort to comply with established guidelines.
Finally, in a given year, approximately 17,000 agencies contribute data to the FBI; however, because of computer problems, changes in record management systems, personnel shortages, or a number of other reasons, some agencies cannot provide data for publication. The FBI appreciates the conscientious efforts made by law enforcement personnel throughout the nation to report accurate and reliable crime data. Their efforts make it possible for the FBI to provide assessments of the nature and type of crime in the United States.