Mass Shooting Statistics Data | The Violence Project

Funded by the National Institute of Justice (award number 2018-75-CX-0023), and recipient of international media attention, the public mass shooter database project includes four phases:

Creation of a comprehensive database of 171 mass public shootings from 1966 to 2019 coded on 100 life history variables, including mental health history, trauma, interest in past shootings, and situational triggers.
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.
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.
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?

The United States has not one gun violence problem, but several. Everyday gun violence claims or changes hundreds of lives each week, disproportionately young Black and Latino men. In 2017 alone, the Centers for Disease Control reported 14,542 homicides by discharge of firearms. About 106 of those deaths were attributable to mass public shootings, according to our data—the highest of any year recorded because of the Las Vegas shooting that claimed an unprecedented 58 lives. The fact that mass shootings account for fewer than 1% of all firearm homicides does not diminish their extraordinary tragedy—mass shootings cause damage far beyond that which is measured in lives lost. It merely highlights that mass shootings are focusing events.

A mass shooting is a variant on "mass murder," but the more generic term lumps together cases that vary along what researchers agree are important dimensions: time, place, and method. 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 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.

There is no universally accepted definition of a mass shooting. We follow the Congressional Research Service definition, which is quite conservative:

“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, proximity to the nearest hospital, and if/how many bullets hit vital organs. However, the number of deaths is the strongest predictor of media coverage, which is necessary to accurately track mass shootings.

By focusing only on public events, we exclude domestic mass shootings (if 50% or more of victims are non-relatives killed in public then we include them). We also exclude mass shootings attributable to underlying criminal activity, and events where a firearm was not the primary means of death. 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:

Other databases focus almost exclusively on spatial and temporal data—the what, when, and where of public mass shootings. Our database is focused on that and everything else, moving us closer to the why and how and, in turn, finding pathways to prevention.

Building The Database

Data Sources Method Research Team

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

Work began in September 2017. On October 1, a gunman on the 32nd floor of the Mandalay Bay resort in Las Vegas opened fire on a crowd of people at a country music festival, killing 58 and injuring hundreds. It was the deadliest mass shooting in history. After that event, our undergraduate students began volunteering to work on the database — at first, for no pay and for no college credit, motivated solely by the need to do something, anything. Over the next two years, they helped us code every mass shooter on 100 different pieces of life history information.

Any coding is a subjective and interpretive process. Informed by existing datasets, the research literature, and frequently asked questions about mass shooters, the Principal Investigators generated a list of variables to be coded and created a codebook to define and detail how to code them. The codebook was then piloted on a small random sample of test cases and refined based on user-experience.

Once the codebook was finalized and coders were trained in its use, the database was populated as follows:

Each mass shooter meeting the inclusion criteria (see definition above) was investigated twice by two separate coders, working independently from each other.
The two resulting datasets were then merged and compared.
Any discrepancies were flagged and reconciled by consensus of the Principal Investigators, who did their own fact-checking and weighed the quality and quantity of the evidence, typically giving precedence to primary source material.
The database was then divided up among the original coders and independently checked again.
Finally, the Database Manager conducted a full and final check, scrutinizing each and every cell.
The Principal Investigators answered any queries resulting from the final check before approving publication. Responsibility for the contents of the database thus lies solely with the Principal Investigators.

This is purposive sample and we present no comparison group. Comparisons are important. For example, 98% of mass shooters are men, but then 90% of all homicide offenders are men. The majority of mass shooters are white, but the racial composition of the sample is comparable to the demographics of the U.S. population overall.

Many of the factors correlated with mass shootings in the database are true of millions of people who never commit mass shootings. People may own guns, have traumatizing childhoods full of violence, reach a crisis point and want to die, think they’ve been victimized, even study other mass shooters, and still not commit a mass shooting. Personality and individual differences cannot be discounted.

There is a low base rate of mass shooters and mass shootings are extreme and rare events—discrete occurrences of infrequently observed phenomena. For this reason, we caution against using the data for predictive modeling or cherry-picking one variable at a time to tell a particular story. For example, we see relatively high rates of mental illness among mass shooters—and rates of thought disorder that are considerably higher than those found in the general population. But this doesn’t mean mass shootings are exclusively caused by mental illness—the vast majority of people with mental disorders are never violent, and are more likely to be victims of violence than offenders. Classifications in the database are based on the available evidence, which sometimes includes demonstrated signs of undiagnosed mental illness and mental health evaluations conducted either before or after the perpetrators committed their attacks.

We have taken every step possible to find and verify sources and to rigorously fact-check the data, but the end result is not perfect. Data privacy laws (rightly) limit full access to official records. The source data were originally gathered for purposes different from our own. Media outlets have their own agendas and biases. Some cases are well reported on, others not so much, resulting in missing data. We also know more about the recent cases, which reflects better reporting over time and more advocacy and awareness around of the topic of mass shootings. For these reasons, users should interpret trends over time with caution.

If you spot a mistake, please tell us so we can correct it. The data are current up to October 1, 2019. There were two mass shootings in the weeks leading up to the release of the database (in a bar in Kansas City and during a Halloween party at an Airbnb rental house in Orinda, California) that do not feature owing to a lack of information. The database will be continually updated and modified as new sources of information become available. Any new perpetrators will be added once per year.

Research Team

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Methodology

What is a Mass Shooting?

Building The Database

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