Data from the nation’s two crime reporting systems, the UCR SRS and NIBRS, were compiled from a sample of states and law enforcement agencies that reported complete, comprehensive crime data to both systems in 2019, the most recent year of available data.
Summary Reporting System (SRS)
The FBI’s Uniform Crime Reporting (UCR) Program Summary Reporting System (SRS) has for decades gathered summary counts of crime reported to law enforcement agencies in the U.S.. The SRS gathers summary counts of crimes including violent crimes, such as rape, robbery, and aggravated assault, and property crimes. In 2019, the SRS included over 18,000 law enforcement agencies nationwide that typically cover more than 90 percent of the U.S. population (Maltz 2006; FBI 2021). The final year of SRS data collection by the FBI was 2020.
The SRS has a series of limitations that may result in undercounting of gun violence reported to law enforcement agencies. The SRS records the type of weapon for only three crime types: robbery, aggravated assault, and murder. For all other types of crime, the SRS either does not specify weapon type or does not include the crime category. For instance, the SRS includes rape but does not provide a breakdown by weapon type. Other crimes that may be committed with a firearm, such as sexual assault or kidnapping, are not included in the SRS (National Incident-Based Reporting System Technical Specification 2020). Further, the SRS records only the single most serious offense associated with each crime, a reporting convention known as the “hierarchy rule” (Federal Bureau of Investigation 2021).
UCR SRS data used in this study were obtained from Jacob Kaplan’s Open ICPSR repository (Jacob Kaplan 2021).
National Incident Based Reporting System (NIBRS)
Development of the National Incident Based Reporting System (NIBRS) commenced in the 1980s with the intention of improving on SRS limitations (National Academies of Sciences, Engineering, and Medicine 2018). NIBRS records offense types that the SRS omits, gathering incident-level data on 52 offenses (Federal Bureau of Investigation 2021). NIBRS also allows a firearm to be reported with more offense types than in the SRS, including sexual assault, rape, kidnapping, and extortion. Further, NIBRS does not impose a hierarchy rule and counts each of the crimes committed in any one incident, and specifies the number of victims in that incident. While NIBRS does measure victim injuries, no field exists for gunshot injuries making determination of a firearm injury impossible.
Starting in 2021, the FBI records police data in NIBRS format only, with substantial reductions in population coverage. As of 2019, NIBRS covers only 53% of the U.S. population (FBI Crime Data Explorer 2020).
NIBRS data were obtained from the FBI’s Crime Data Explorer, which features tables counting incidents, offenses, and victims. Incidents count the number of distinct incidents of a crime that occur and allow for a crime incident to be classified as multiple types of crimes. The victim tables expand on the crime incident to link multiple victims to a crime incident.
Sample selection
To compare UCR SRS and NIBRS measurements of gun violence, states whose population coverage in NIBRS is at or above 98% population coverage from 2015–2019 statewide were selected. These states include Colorado, Kentucky, Michigan, South Carolina, Tennessee and Virginia and comprise approximately 11% of the U.S. population. Law enforcement agencies within each state were then matched between UCR SRS and NIBRS sources to compare measures across the sources. All measures are calculated using data from 2019, the most recent year of data available both from the SRS and NIBRS.
Measures
Several measures were constructed from incident-level NIBRS data to be directly comparable to aggregated UCR SRS data (Law Enforcement Support Section (LESS) Crime Statistics Management Unit (CSMU) 2013). The UCR SRS reports a robbery incident count and aggravated assault as a victim count. NIBRS data were adjusted to match SRS conventions.
To calculate the number and proportion of crimes committed with a firearm, NIBRS weapons categories including handgun, rifle, shotgun, other firearm, and automatic firearm categories were counted if they were attributed to a crime incident for robberies, or a crime victim or victims for aggravated assault. The corresponding UCR SRS measure is used for comparison. The rates of assaults and robberies committed with a firearm are compared by calculating the proportion of robbery and assault committed with a firearm using the denominator of total robberies and assaults. To identify multiple offenses in NIBRS, the unique incident number of each firearm crime was matched to all other offenses associated with the same incident identifier.
To measure additional firearm crime captured in NIBRS, two categories were constructed. NIBRS crime categories with a firearm include kidnapping, extortion, rape, sexual assault, and weapons law offenses in addition to aggravated assault and robbery that both NIBRS and SRS both capture. However, in NIBRS, many of the additional firearm categories are not frequently populated. For this reason, rape and sexual assaults are combined into a single category and kidnapping is combined with exhtortion to form an additional single cateogry.
Comparisons are made at the state unit of analysis as well as the FBI’s population group variable, which is a categorical classifier describing the size and nature of the population each law enforcement agency serves. Population groups under 100,000 residents are combined into one measure as they are less frequent than large city reporting and have similar reporting patterns.
Analysis
NIBRS and SRS count and proportion measures of robbery and aggravated assault with a firearm are compared to understand to what extent the SRS may undercount gun violence in 2019, the most recent year of available NIBRS data. Proportions of robbery and aggravated assault with a firearm are calculated in reference to a denominator of all aggravated assaults and robberies, which may be committed without a firearm or with another weapon, such as a knife. These proportions are compared using chi-squared tests of proportion to assess whether the measures differ statistically at the 0.05 level of significance.
Because the SRS suppresses counts of crime when a more severe crime also occurs according to the hierarchy rule, the SRS may underestimate gun violence. To assess the extent of variation from this reporting convention, offenses that would have been suppressed in the SRS are tabulated.
To assess agency coding, the proportion of crimes involving firearms by source are compared among agencies serving a population larger than 100,000. If the distribution shows a large proportion of agencies with exactly no difference between their SRS and NIBRS reports, agencies may not have fully adopted reporting multiple crimes per incident. Similarly, if only a handful of agencies account for the majority of the NIBRS and SRS difference, the problem may be individual agency NIBRS adoption. If agencies on average tend to report additional offenses with stable proportions of firearm use, as agencies have experience reporting weapons for these crimes for decades, this is suggestive that NIBRS may improve existing coding of offenses. Law enforcement agency population groups are also used to assess if crime reporting differs by population type and size.
Next, categories of gun violence that are not measured in the SRS system but are tracked in NIBRS are tabulated to understand the extent to which underestimation of gun violence in the SRS may be a function of more limited SRS crime categories.
Analysis was performed in R 4.1.0. This study was considered exempt from review by the institutional review board at the University of Michigan.