the trends in delinquency over the past several decades, and considers what forecasts can be made about juvenile crime.

SOURCES OF DATA

Three ways in which crime is often measured are arrest statistics, victim reports of crimes, and self-reports of offenses. These sources may yield different crime rates and trends. Each source has advantages and drawbacks, and each alone gives an incomplete picture of crime. In this section, we discuss these sources of data and their strengths and weaknesses.

Arrest Data

A common way of measuring crime is to use the Uniform Crime Reports (UCR), which are compiled from data on crimes known to the police and on arrests that are reported annually to the Federal Bureau of Investigation (FBI) by police agencies around the country. Data have been collected by the FBI since 1930, allowing the study of crime and arrest trends over time. The UCR provide crime counts for the United States as a whole, as well as for regions, states, counties, cities, and towns. In addition, the UCR provide data on, among other things, crimes known to the police, crimes cleared by arrest, and characteristics of persons arrested. However, UCR reporting is voluntary, and the total number of reporting police agencies varies from year to year. The accuracy and completeness of the data are affected by the voluntary nature of UCR reporting (Maltz, 1999). In some years, data from one or more entire states have been unavailable. For example, from 1988 to 1991, no usable data were obtained from either Florida or Kentucky (Federal Bureau of Investigation, 1998). Coverage within states also varies from year to year. The FBI imputes information when none has been reported. Because many of the tables in the published UCR, including the breakdown by age, are based on whichever agencies report in a given year and not on a nationally representative sample, caution must be used in making generalizations to all young people in the United States based on UCR data. This is particularly true with regard to analyses regarding race, because the racial makeup of the areas covered by reporting agencies may not reflect the racial makeup of the country.

Data in the UCR are reported by offense for 28 different offenses (for definitions of offenses used in the UCR, see Appendix A). The most information is reported on what are termed index (or part I) crimes— eight crimes that make up the crime index, which is used “to gauge fluctuations in the overall volume and rate of crime reported to law enforce



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