There is a “dark figure” of unknown crime about which we remain ignorant. To attempt to understand how and why cases “drop” out of the system, we use different sources of statistics, which have different strengths and weaknesses. Official statistics come from official agencies; for example, the Uniform Crime Reports is an annual report of crimes known to the police and the characteristics of persons arrested for “index crimes.” Unofficial data come from several sources, including the National Crime Victimization Survey and self-report studies, as well as cohort and participation studies. There is agreement among measures that the most serious offenders come from lower socioeconomic classes; those who go through processing are not representative of the general population; and urban dwellers, the poor, young persons, males, and minority group members are more likely to be arrested and processed in numbers incommensurate to their populations.