The purpose of the study was to investigate the role and impact of forensic science evidence on the criminal justice process. Specifically, for this project, the National Institute of Justice established four goals and objectives:
- Estimate the percentage of crime scenes from which one or more types of forensic evidence is collected;
- Describe and catalog the kinds of forensic evidence collected at crime scenes;
- Track the use and attrition of forensic evidence in the criminal justice system from crime scenes through laboratory analysis, and then through subsequent criminal justice processes; and
- Identify which forms of forensic evidence contribute most frequently (relative to their availability at a crime scene) to successful case outcomes.
The study utilized a prospective analysis of official record data that followed criminal cases in five jurisdictions (Los Angeles County, California; Indianapolis, Indiana; Evansville, Indiana; Fort Wayne, Indiana; and South Bend, Indiana) from the time of police incident report to final criminal disposition. Sites were selected to represent city, county and state crime laboratory services. The data were based on a random sample of the population of reported crime incidents between 2003 and 2006, stratified by crime type (aggravated assault, burglary, homicide, rape, and robbery) and jurisdiction. A total of 4,205 cases were sampled including 859 aggravated assaults, 1,263 burglaries, 400 homicides, 602 rapes and 1,081 robberies. Descriptive and impact data were collected from three sources: police incident and investigation reports, crime lab reports, and prosecutor case files. An unique case identifier linked police incident and crime lab reports and, for most crime incidents, connected the case with the prosecutors' database. For cases that could not be linked through the unique identifier number, suspects name, race/ethnicity, and birth date were used to connect the case with prosecutor data.
Various forensic variables were used for both descriptive and outcome analyses. Data were collected on various crime scene evidence types (biological, latent prints, pattern evidence, firearms/weapons, natural synthetic materials, generic objects, electronic/printed data, trace, drugs, and other) and substrates. Furthermore, the study explored the effect of forensic evidence on five different case outcomes, including:
- whether a reported crime incident resulted in an arrest,
- whether a case arrest was referred to the district attorney,
- whether an arrested suspect(s) was formally charged,
- whether a prosecuted defendant was convicted, and
- sentence length for incarcerated offenders.
A total of 4,205 cases from five jurisdictions (Los Angeles County, California; Indianapolis, Indiana; Evansville, Indiana; Fort Wayne, Indiana; and South Bend, Indiana) were sampled including 859 aggravated assaults, 1,263 burglaries, 400 homicides, 602 rapes, and 1,081 robberies. The data were based on a random sample of the population of reported crime incidents for the year 2003, stratified by crime type and jurisdiction. Crime incidents for the year 2003 were used so that cases would have complete data, including final dispositions. Due to the relatively low numbers of homicides and rapes committed annually, as well as the greater likelihood of forensic evidence for these two crimes, reported incidents for homicide and rape were over-sampled for Los Angeles and Indianapolis.
Similarly, due to the lower number of crime incidents in the three smaller Indiana sites (Evansville, Fort Wayne; and South Bend) as well as changes in data management systems in Evansville, additional years were included in the sample selection process for these sites. Specifically, in Evansville, because of a change in data management systems, effective mid-September 2003, and due to the relatively low numbers of homicides and rapes committed annually, all homicides occurring from mid-September 2003 through December 2006 and all rapes occurring from mid-September 2003 through December 2005 were reviewed to achieve the desired sample size. Likewise, due to the change in data management systems, aggravated assault, burglary and robbery cases in Evansville were sampled from incidents occurring after September 2003 and during all of 2004. In Fort Wayne and South Bend, due to the relatively low numbers of homicides committed annually, all homicides occurring during 2003 and 2004 were reviewed to achieve the desired sample size.
All reported incidents of aggravated assault, burglary, homicide, rape, and robbery that occurred in Los Angeles County, California; Indianapolis, Indiana; Evansville, Indiana; Fort Wayne, Indiana; and South Bend, Indiana; between 2003 and 2006.
police incident and investigator reports
crime lab reports
prosecutor case files
administrative records data
The data contain a total of 175 variables including site, crime type, forensic variables, criminal offense variables, and crime dispositions variables. Specifically, forensic variables include whether evidence was collected at crime scene, types of evidence and substrates collected at crime scene, whether evidence was submitted to crime lab, types of evidence and substrates submitted to lab, and whether evidence was examined in lab. Other forensic variables include whether evidence links suspect to victim and/or crime scene and whether tangible evidence was collected at crime scene. Criminal offense variables include time from incident to report, time from incident to arrest, victim characteristics, suspect characteristics, number of eyewitnesses, number of victims, victim report to police, witness report to police, victim/suspect relationship, and victim receipt of medical treatment. Crime dispositions variables include suspect arrest, district attorney referral, case charged, case conviction, sentence type, sentence length, attorney type, plea, suspect apprehended within ten minutes of the crime, type of arrest technique, number of prior arrests, and number of prior convictions.