The source of the data is police investigation files. Victims and offenders are those identified by police investigation.
Victims are the people who died.
Offenders include all those known to the police, whether or not arrested. The Murder Analysis Report is a one-page summary of each homicide, which has been maintained since 1965 by the Crime Analysis Unit of the Chicago Police Department. When the coders have a question about the MAR information, or they need clarification about what happened in a particular case, officers in the Crime Analysis Unit advise them as to the correct codes and definitions. All coding and data entry are carried out in the Crime Analysis Unit. Coders are supervised closely and trained continuously.