We examined what constitutes a “mass shooting” according to each of four data sources: Gun Violence Archive (GVA), Everytown for Gun Safety (EGS), Mother Jones Investigation, and the Federal Bureau of Investigation’s (FBI's) Supplementary Homicide Report (SHR). We also included the FBI's record of active shooter incidents. “Active shooter” and “mass shooting” are not synonymous, but the FBI’s record of active shooter incidents is the closest to federal data collection on events that resemble mass shooting incidents. First, we completed descriptive analyses of mass shootings that occurred in 2017 to illustrate the difference in statistics that may be reported depending on which dataset is referenced. We then evaluated database overlap by examining the events that were common to all or multiple databases. Finally, we applied one mass shooting definition- based on 4-person fatality count- to each of the 4 mass shooting data sources – excluding the active shooter report - to determine whether discrepancies among databases is accounted for when the same victim count-related definition is applied.

Data sources

Data were obtained from four sources: Everytown for Gun Safety (2017), Gun Violence Archive (2015), the FBI’s Supplementary Homicide Report (Federal Bureau of Investigation 2018), and “U.S. Mass Shootings, 1982 – 2018” from Mother Jones’ Investigation (Mark Follman and Pan 2018). Information from the FBI’s Active Shooter Report for 2016–2017 (Schweit 2016) was also included in our study. Specifically, data on shootings that occurred in 2017 were collected, since this is the most recent year for which data are available from all sources. Definitions of “mass shooting” and collection requirements for each of the five databases are described below.

Everytown for gun safety

In 2018, Everytown for Gun Safety released a report on mass shootings that occurred from 2009 to 2017. According to Everytown, a mass shooting is an event in which at least four individuals are killed with a firearm; the fatality requirement excludes the shooter (Everytown for Gun Safety 2017). This definition is based on the 2005 FBI report on serial murder, and information included in Everytown’s analysis is pulled from media reports, official records, and the FBI’s Supplementary Homicide Report. When available, the EGS report included information on category of firearm(s) used and how the firearm(s) were acquired, whether the perpetrator was legally prohibited from owning firearms or could have been subjected to a gun violence restraining order (GVRO) law, and whether the location of the incident was a designated “gun-free” zone. Everytown collected information on incidents that occurred in both private and public settings, regardless of motive. Therefore, domestic violence incidents that occurred in the home may be included, as well as incidents of group violence pertaining to gang affiliation or drug trafficking.

Gun violence archive

Gun Violence Archive (GVA) is a non-profit organization established in 2013. The goal of GVA is to collect information regarding gun violence and to make this information available to the public online for use in gun violence and safety research. GVA defines a mass shooting as an event where four or more individuals are shot, but not necessarily killed (Gun Violence Archive 2015). Although this threshold is not supposed to include the shooter, exploratory analysis found that some of the shootings recorded did include the suspect in the count of people injured or killed. GVA differs from other datasets in that gang- and drug-related incidents were included in the GVA dataset. Data regarding mass shooting events were gathered via media and police sources, and links to media articles are provided for most mass shootings listed on the website. Information regarding shooter demographics, type of firearm, and venue of shooting may be listed on an individual shooting’s page or in links related to the shooting, but GVA does not necessarily track this information.

FBI’s supplementary homicide report

The FBI’s Uniform Crime Reporting (UCR) program was created in 1929 to be a collection of national crime statistics, and many researchers use the UCR to analyze crime. Reporting crime incidents to the UCR is not mandatory, and so UCR data rely on crime reporting from individual police agencies. Not all states choose to report to the UCR, and not even all agencies within a state choose to report crime information. The Supplementary Homicide Report (SHR) is a subset of the UCR that provides extensive details on homicides that have been reported, such as perpetrator demographics, weapon information, and incident circumstances. Although the FBI does not specifically label incidents as “mass shootings,” a few studies have been completed which use FBI SHR data to examine mass shooting trends (Reeping et al. 2019; Fox and Fridel 2016). For this reason, we have included an examination of the FBI SHR in this paper, with a definition of 4 or more killed in an incident as used in other publications.

Mother Jones’ investigation: US mass shootings, 1982–2018

The Mother Jones “Mass Shootings in America” is an open-source database that was created after the 2012 movie theater shooting in Aurora, Colorado. When Mother Jones initially collected data on mass shootings in 2012, they made use of the FBI’s definition for mass murder - four or more individuals killed in a single attack in a public place (Mark Follman and Pan 2018). However, the FBI’s definition was adjusted in 2013 – via mandate by President Obama - to include three or more people killed indiscriminately, and Mother Jones adjusted its definition and included qualifying incidents accordingly. Mother Jones does not include gang-related crimes or robbery in its dataset, and the shootings included were carried out indiscriminately by one individual in a public place. Information on venue, shooter demographics, shooter’s history of mental illness and weapon details are recorded in the Mother Jones dataset as well. Mother Jones admits that their data are narrowly focused and not as useful as other sources for studying gun violence as a whole, but they claim that their research provides insight at the “distinct phenomenon” of mass shootings (Mark Follman and Pan 2018).

FBI active shooter incidents in the United States in 2016 and 2017

In 2014, the FBI released a report detailing active shooter incidents that occurred between 2000 and 2013. The report was generated as an attempt to prepare law enforcement agencies for such situations and provide information that may help prevent incidents from occurring; an updated report has been released every 2 years since the release of the initial report in 2013. The FBI report defines an active shooter incident as a case in which at least one person is “actively engaged in killing or attempting to kill people in a populated area” (Schweit 2016). There is no set number of victims needed for an incident to be included in the FBI’s active shooter analysis. The FBI Active Shooter Report does not include shooting incidents that pertained to drug or gang involvement and accidental firearm discharges in public spaces were omitted from their analysis.

Analytic strategy

For each dataset, descriptive statistics were generated using Stata 14.2 software to illustrate the fatality and victim counts and averages from mass shootings that occurred in 2017 (StataCorp 2015). The goal of the analysis was to highlight the differing findings that result when we compare multiple data sources, which all claim to track mass shootings (or in the FBI’s case, active shootings). After determining descriptive statistics for each dataset, we then excluded incidents with less than 4 people fatally injured; this is the definition used by Everytown and is the strictest victim-count definition out of the data sources examined. The purpose of this exclusion was to investigate whether applying the same definition to all the datasets would cause them to have similar numbers of mass shooting frequency and discuss any differences that may persist after the one definition is applied to all sources. To further highlight differences in the incidents captured among the examined databases, we used the R programming software to generate Venn Diagrams (Larsson 2019; R Core Team 2019).