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Index crimes are the eight crimes the FBI combines to produce its annual crime index. These offenses include willful homicide, forcible rape, robbery, burglary, aggravated assault, larceny over $50, motor vehicle theft, and arson. In order to compare statistical information on a national basis it was necessary to come up with a common definition for crime comparison. The index seeks to overcome differences in individual state statues, that would ignore how the individual is charged, and create a standardized definition of crime classification. This was done through defining serious and non-serious offenses. Part I crimes are comprised of serious felonies and Part II crimes are comprised of non serious felonies and misdemeanors. Together these two types of classifications make up the crimes reported in the Uniform Crime Reports (UCR).
The reporting of offenses known is limited to the selected crime classifications because they are the crimes most likely to be reported and most likely to occur with sufficient frequency to provide an adequate basis for comparison. Arson was not originally part of the crime reporting process, but was added to the Crime Index in October 1978. With the passage of the Anti-Arson Act of 1982, Arson was permanently designated as reportable.
The Crime Index total is the sum of selected offenses used to measure crime rates and their fluctuations reported to law enforcement. The offenses included in the Crime Index total are the violent crimes of Murder and Nonnegligent Manslaughter, Forcible Rape, Robbery, and Aggravated Assault, and the property crimes of Burglary, Larceny-theft, and Motor Vehicle Theft. Arson is excluded due to inconsistent reporting.
Participation in the National UCR Program is strictly voluntary, however, law enforcement agencies representing roughly 95% of the nations population participate. In reporting crimes, agencies are instructed to follow a 'hierarchy rule", so that when several offenses are committed at the same time and place, after classifying all Part I offenses, only the highest scoring offense is scored and the rest are ignored.
For more information on Index Crimes and Uniform Crime Reports, see: