Because the BJS designed the NCVS to complement the UCR Program, the two programs share many similarities. As much as their different collection methods permit, the two measure the same subset of serious crimes, defined alike. Both programs cover rape, robbery, aggravated assault, burglary, theft, and motor vehicle theft. Rape, robbery, theft, and motor vehicle theft are defined virtually identically by both the UCR and the NCVS. (Although rape is defined analogously, the UCR Program measures the crime against women only, and the NCVS measures it against both sexes.)
There are also significant differences between the two programs. First, the two programs were created to serve different purposes. The UCR Program’s primary objective is to provide a reliable set of criminal justice statistics for law enforcement administration, operation, and management. The BJS established the NCVS to provide previously unavailable information about crime (including crime not reported to police), victims, and offenders.
Second, the two programs measure an overlapping but nonidentical set of crimes. The NCVS includes crimes both reported and not reported to law enforcement. The NCVS excludes, but the UCR includes, homicide, arson, commercial crimes, and crimes against children under age 12. The UCR captures crimes reported to law enforcement but collects only arrest data for simple assaults and sexual assaults other than forcible rape.
Third, because of methodology, the NCVS and UCR definitions of some crime differ. For example, the UCR defines burglary as the unlawful entry or attempted entry of a structure to commit a felony or theft. The NCVS, not wanting to ask victims to ascertain offender motives, defines burglary as the entry or attempted entry of a residence by a person who had no right to be there.
Fourth, for property crimes (burglary, theft, and motor vehicle theft), the two programs calculate crime rates using different bases. The UCR rates for these crimes are per capita (number of crimes per 100,000 persons), whereas the NCVS rates for these crimes are per household (number of crimes per 1,000 households). Because the number of households may not grow at the same rate each year as the total population, trend data for rates of property crimes measured by the two programs may not be comparable.
In addition, some differences in the data from the two programs may result from sampling variation in the NCVS and from estimating for nonresponse in the UCR. The BJS derives the NCVS estimates from interviewing a sample and are, therefore, subject to a margin of error. The BJS uses rigorous statistical methods to calculate confidence intervals around all survey estimates. The BJS describes trend data in the NCVS reports as genuine only if there is at least a 90-percent certainty that the measured changes are not the result of sampling variation. The UCR Program bases its data on the actual counts of offenses reported by law enforcement agencies. In some circumstances, the UCR Program estimates its data for nonparticipating agencies or those reporting partial data.
Apparent discrepancies between statistics from the two programs can usually be accounted for by their definitional and procedural differences or resolved by comparing NCVS sampling variations (confidence intervals) of those crimes said to have been reported to police with UCR statistics.
For most types of crimes measured by both the UCR and NCVS, analysts familiar with the programs can exclude from analysis those aspects of crime not common to both. Resulting long-term trend lines can be brought into close concordance. The impact of such adjustments is most striking for robbery, burglary, and motor vehicle theft, whose definitions most closely coincide.
With robbery, the BJS bases the NCVS victimization rates only on robberies reported to the police. It is also possible to remove UCR robberies of commercial establishments such as gas stations, convenience stores, and banks from analysis. When users compare the resulting NCVS police-reported robbery rates and the UCR noncommercial robbery rates, the results reveal closely corresponding long-term trends.
Each program has unique strengths. The UCR provides a measure of the number of crimes reported to law enforcement agencies throughout the country. The UCR’s Supplementary Homicide Reports provide the most reliable, timely data on the extent and nature of homicides in the Nation. The NCVS is the primary source of information on the characteristics of criminal victimization and on the number and types of crimes not reported to law enforcement authorities.
By understanding the strengths and limitations of each program, it is possible to use the UCR and NCVS to achieve a greater understanding of crime trends and the nature of crime in the United States. For example, changes in police procedures, shifting attitudes towards crime and police, and other societal changes can affect the extent to which people report and law enforcement agencies record crime. NCVS and UCR data can be used in concert to explore why trends in reported and police recorded crime may differ.