Methodology

Home > Methodology

Initially funded by the National Institute of Justice (award number 2018-75-CX-0023), and recipient of global media attention, the mass shooter database project included four phases:

  1. Creation of a comprehensive database of all mass public shootings from 1966 to 2020 coded on nearly 200 life history variables, including mental health history, trauma, interest in past shootings, and situational triggers.
  2. Examination of community-level socio-ecological factors of where mass public shootings take place, including, but not limited to, crime rates, measures of social inequality, community mobility, availability of mental health resources, and prevalence of gun stores.
  3. In-depth life history interviews with living mass shooters who are currently incarcerated and follow-up interviews with key stakeholders (e.g., family members, first responders, survivors, experts) in the communities where shootings took place.
  4. Dissemination of findings, creation of this public website, and implications for evidence-based prevention strategies.

The work is guided by rigorous ethical protocols from the Institutional Review Board at Hamline University and a steadfast commitment to no notoriety for mass shooters.

What is a Mass Shooting?

According to CDC mortality data, there were over 45,000 firearm-related deaths in the United States in 2020, not to mention the many thousands more survivors, families, and others changed forever because of gun violence. There is not one gun violence problem in this country but multiple gun violence problems that demand our attention and action, including:

  1. Suicide by firearm;
  2. Everyday gun violence (i.e., interpersonal conflicts and shootings related to gangs/groups or underlying crime);
  3. Domestic violence;
  4. Unintentional shootings;
  5. Police-involved shootings;
  6. Mass shootings.

Some of these problems are more prevalent than others and they each have their own victim and offender profiles, underlying mechanisms, and potential solutions.

There is no universally accepted definition of a mass shooting. For a long time mass shootings were studied as a variation on "mass killing", the generic term for multiple homicides that vary along what researchers agree are important dimensions: time, place, and method. For instance, someone who kills their victims in separate events is different from someone who kills them all at once. A person who kills in public is different from a person who kills in private (especially when private victims tend to be family members or intimate partners); and different still from a contract killer, bank robber, or gang member who kills in the commission of another crime. And an arsonist or bomber is different from a shooter.

Some sources define a mass shooting as any incident in which three or more victims were shot and killed, not including the perpetrator, while others use a threshold of four or more killed. Other sources define a mass shooting as any incident in which four or more people were shot or injured, which greatly increases the incident count.  Some sources include domestic violence, gang conflict, drug trade disputes, or robberies in their numbers. At The Violence Project, we focus on mass public shootings, defined by the Congressional Research Service as follows:

“a multiple homicide incident in which four or more victims are murdered with firearms—not including the offender(s)—within one event, and at least some of the murders occurred in a public location or locations in close geographical proximity (e.g., a workplace, school, restaurant, or other public settings), and the murders are not attributable to any other underlying criminal activity or commonplace circumstance (armed robbery, criminal competition, insurance fraud, argument, or romantic triangle).”

We acknowledge the limits of this definition. Every mass casualty event is a tragedy and many factors influence whether a threshold of four or more people killed is reached, including the accuracy of the shooter, the type and caliber of weapon used, the number of rounds fired, the actions of first responders, proximity to the nearest hospital, and if/how many bullets hit vital organs. Any cut point is arbitrary, but this remains a widely agreed upon standard. Further, the number of deaths is the strongest predictor of media coverage, which we use to help build our database.

By focusing only on mass public shootings, we exclude domestic homicides (although if 50% or more of victims are non-relatives killed in public then we include them). We also exclude mass shootings attributable to gangs or underlying criminal activity (e.g., a robbery or drug deal gone bad). A broader definition with a threshold of fewer deaths, non-fatal shootings, or any means or motive would certainly yield more cases. For examples, see:

Source Dates Casualties Location Exclusions
Violence Project 1966–

present

 4+ killed

(excludes shooter)

 Public When victims are exclusively family or friends;

shootings related to gangs or underlying crime
Gun Violence

Archive

 2014–

present

 4+ shot Any None
AP/USA Today/

Northeastern

University

 2006–

present

 4+ killed

(excludes shooter)

 Any None
Everytown for Gun Safety 2009–

2018

 4+ killed Any None
Stanford Mass Shootings of America 1966–

2016

 3+ shot

(excludes shooter)

 Any Shootings related to gangs, drugs, or organized crime
Washington Post 1966–

present

 4+ killed

(excludes shooter)

 Public When victims are exclusively family or friends;

shootings related to gangs or underlying crime
Mother Jones 1982–

present

 3+ killed

(4+ killed ≤ 2013)

 Public “Conventionally motivated crimes” such as gang violence and armed robbery

Our database was primarily built to explore the life histories of mass shooters, not simply track mass shootings. Our main unit of analysis is the shooter, not the shooting.  This means that in addition to the what, when, and where of mass public shootings, we are focused on the why and how, and the victims of these crimes, hopefully moving us closer to a world without mass shootings.

Building The Database

Data Sources
Data Sources

To build the database, we used the following:

Primary Sources:

  • Written journals / "manifestos" / suicide notes etc.
  • Social media and blog posts
  • Audio and video recordings
  • Interview transcripts
  • Personal correspondence with perpetrators

Secondary Sources (all publicly available):

  • Media (television, newspapers, magazines)
  • Documentary films
  • Biographies
  • Monographs
  • Peer-reviewed journal articles
  • Court transcripts
  • Law Enforcement records
  • Medical records
  • School records
  • Autopsy reports

Census Variables:

  • U.S. Census Bureau
  • FBI Uniform Crime Reports
  • Google Maps
Method
Research Team
Updates

Want to Know More About What We Do at the Violence Project?

LEARN MORE ABOUT THE VIOLENCE PROJECT