These data collection systems utilize different methods of measuring criminal behavior. The UCR relies on official data that have been collected and reported by law enforcement agencies. The NCVS and other surveys discussed in this report are large-scale social surveys that rely on self-reports of offenses or victimization.

Although these data collection systems do many things right, they are, like any such system, beset with the methodological problems of surveys in general as well as particular problems associated with measuring illicit, deviant, and deleterious activities. Such problems include nonreporting and false reporting, nonstandard definitions of events, difficulties associated with asking sensitive questions, sampling problems such as coverage and nonresponse, and an array of other factors involved in conducting surveys of individuals and implementing official data reporting systems.

Compounding these problems are the recent interest in rare crime events, such as violent crimes committed by youth and hate crimes; the need for attention to vulnerable subpopulations, such as very young and school-age children and disabled, elderly, and immigrant populations; and a focus on small- or local-area estimates of crime and victimization. Congress periodically requires the U.S. Department of Justice to develop new research or data collection efforts to measure crime victimization in specific populations and for small areas. Understanding victimization and offending in these subgroups, however, can be particularly difficult.

In general, criminal victimization is a relatively rare event—that is, in any given reference period, the majority of respondents do not report any victimization. Very large general population samples are therefore required to accurately characterize the population of offenders and victims, and detailed subgroup analyses can be problematic. Some important subgroups may not be covered at all (e.g., homeless people), and smaller research studies of crimes against these subgroups often have problems of statistical power because of small sample sizes in most cases. For many hard-to-identify subpopulations, such as people with disabilities and abused children, there is no large, well-defined group from which to draw a sample for measuring victimization—in other words, a sampling frame. This, as well as more conventional problems associated with interviewing crime victims, presents substantial design and analytical difficulties.

Official data such as UCR arrest data have a different set of problems. Foremost among them is that most crimes are not reported to the police, and only a small proportion of those that are reported result in an arrest. Increases or decreases in reports or in arrests for certain offenses, such as

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