For this series, The Washington Post collected data on crimes reported to 20 local police departments and compiled by the Federal Bureau of Investigation from 1977 to 1997. Known as the Uniform Crime Reports, or UCR, the data include the number of homicides, rapes, robberies, aggravated assaults, burglaries, larcenies and car thefts.

Additional data were collected from state and local police, the U.S. Census Bureau, the Bureau of Justice Statistics, marketing research firms, universities and nonprofit organizations. More than 70 interviews were conducted.

Criminologists caution that police departments may vary in the accuracy of their record-keeping and the way they categorize crimes. For example, one department might define a home break-in as an attempted burglary, while another might call it vandalism.

In the latter case, the incident would not have to be reported in UCR totals for serious crimes.

The number of crimes reported to police departments probably understates the number of crimes committed. For example, some victims of sexual assault may not file a report out of fear or embarrassment.

Increases or decreases in a particular category of crime may reflect changes in police emphasis rather than differences in the actual incidence of the crime.

Still, crime experts agree that UCR data are the best available to assess the changes in crime over time, and to compare crime in different jurisdictions.

Contributors to this series included staff writer Steve Vogel, assistant polling director Claudia Deane and database specialist Jo Craven.