December 21, 2022
Mass Shootings: Definition, Statistics & Biases
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Despite being considered one of the “largest problems facing the country” there is surprisingly no consensus on what the definition of a mass shooting actually is.

A cursory online search of the phrase “mass shootings definition” will yield no less than a dozen conflicting results, depending on a myriad of factors, such as the shooter’s intention, the location of the shooting, the number of injuries, and the number of fatalities. Because of this flexible and imprecise definition, you could be led to believe that the United States experiences an average of 19 mass shootings every year by one researcher, while another site will tell you that a mass shooting happens nearly every day.

By any metric, the number of mass shootings in America is unacceptably high. However, without a consistent definition of mass shootings, it is becoming more difficult to gather reliable evidence to start presenting solutions to this problem.

Thus, it’s worth exploring how conflicting statistics are shaping public understanding of mass shootings and creating biases that may be preventing progress on this front. Or, put more succinctly by Injury Epidemiology: “Establishing definitive guidelines for a mass shooting definition could improve research credibility when presenting evidence to policymakers.”

Mass Shootings Definition

ZeroEyes defines a mass shooting as any incident where four or more people, excluding the shooter, are injured or killed in a gun-related attack. There is no focus placed on the shooter’s intention, relationship to the victims, or location of the attack. The key factor in this definition is the casualty threshold, with the crucial distinction that it only accounts for both injuries and fatalities. Thus, an incident defined as a mass shooting could conceivably result in only injuries and no loss of life. 

We have derived this definition to coincide with how the Federal Bureau of Investigation (FBI) defines mass shootings. As the chief government agency tasked with responding to and investigating mass shootings across the United States, the FBI is the most credible and consistent source for terminology, resources and information on the subject. The same definition has also been adopted by the Gun Violence Archive (GVA) – a trustworthy site for statistics on gun-related attacks. 

Though we use the same definition of mass shootings as two of the foremost authorities on the subject, it is important to remember that there is no federally-recognized definition, so not all sources of information use comparable language. 

Conflicting Definitions of “Mass Shooting”

Our mission is to prevent gun-related violence of all kinds, so for the definition of “mass shooting,” ZeroEyes does not include any qualifications beyond the quantity threshold of four injuries or fatalities, excluding the shooter.

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See How ZeroEyes Helps Prevent Mass Shootings

Learn more about the ZeroEyes mission and how A.I. gun detection can play a key role in proactive protection against gun-related violence.


Other organizations take a more targeted approach with their definition, which can sculpt the data to fit a specific bias. While perhaps created with good intentions, the research of these organizations is often presented in incompatible ways that create skewed statistics. 

Below are several common qualifiers that create data bias when defining mass shootings. 

Defining Mass Shooting by Fatalities

Several years ago at Thurston High School in Springfield, OR, an active shooter opened fire. Two individuals lost their lives and 25 people were wounded. Incredibly, by the metrics of Everytown Research this would not be considered a mass shooting.

Everytown Research, among many others, uses fatalities to qualify their definition of a mass shooting. In their specific definition, an incident must include four or more people being shot and killed, excluding the shooter. 

With this strict, fatality-driven definition of mass shootings, it is no wonder their reporting numbers are so low compared to sources like the FBI and the Gun Violence Archive. You may be interested in exploring Everytown’s methodology, yet we would argue most reasonable Americans would qualify the Thurston High School shootingan attack where 27 individuals were struck with bulletsas a mass shooting event. 


Defining Mass Shooting by Context

In October of 2022 a Chicago man fired 13 rounds, striking five people. Four of them were taken to the hospital. Three died there. This is not considered a mass shooting by the Mother Jones database because the incident was deemed as gang violence.

Mother Jones US Mass Shootings Database, another frequently-cited source in the mass-shooting discourse, has opted to not include gang-related attacks and other mass shootings that occur during criminal activities in their reporting. These qualifiers are even stricter than Everytown Research, as they not only adopt a four-fatality threshold, but also omit incidents that are deemed to be part of drug activities, robberies and other illicit actions. Their focus solely on indiscriminate public killings presents an extremely skewed view of mass shootings that does not align with the severity of the problem.

Defining Mass Shooting by Location

This past Thanksgiving in Spring Branch, TX, a man entered his ex-wife’s home and opened fire. Two individuals were left dead and two more were injured. This incident would have been omitted by the Rockefeller Institute for Government, which  defines a mass shooting as “an incident of targeted violence carried out by one or more shooters at one or more public or populated locations,” which leaves out a large contingent of mass shootings: domestic attacks that occur inside the home. 

The Educational Fund To Stop Gun Violence (EFTSGV) reports that a staggering two thirds of mass shootings are linked to domestic violence. The omission of shootings in private locations by the Rockefeller Institute for Government and other similarly-minded sites surely damages the reliability of their data for use in a cohesive discussion about mass shootings. 

Bias in Mass Shooting Data

As we’ve illustrated above, every credible source has a nuanced definition of mass shootings. With different parameters for what they consider a mass shooting, it can be like comparing apples to oranges when trying to construct a reliable portrait of the problem at hand. Problems arise when two equally valid facts seemingly contradict each other and their respective bias becomes apparent. 

Take for example the data cited in the debate around gun-free zones:

An Example of Conflicting Statistics About Mass Shootings

In 2018, a study by the Crime Prevention Research Center (CPRC) reported that 97.8% of mass shootings happen in gun-free zones. This statistic was touted as evidence that gun-free zones are ineffective and create an easy target for would-be attackers. And understandably so: any reasonable person reading that statistic would be struck with the thought that gun-free zones are ineffective if nearly 98% of mass shootings occur there. However, three years later in 2021, Everytown Research posted a study claiming that 61% of mass shootings happened inside of homes

Due to the widely varying definition of “mass shooting,” both of these statistics are factually true.

Because there is no federally-defined definition of mass shootings, the researchers each chose a different definition that highlighted their bias. Both defined a mass shooting as an event with four or more fatalities, but the CPRC omitted mass shootings that coincided with criminal activity, drug involvement or domestic violence. This leaves out nearly every attack that took place inside the home – which Everytown Research claimed represented 61% of mass shootings.  

Though the CPRC researcher presented methodology that defended their definition, it is an incredibly narrow definition of mass shooting, seemingly tailored to attack the efficacy for gun-free zones. You can learn more about gun-free zones on our blog. 

Mass Shootings vs. Mass Killings

Though often erroneously used interchangeably, the terms “mass shootings” and “mass killings” represent very different ideas in the realm of research and statistics. It is important that one has an understanding of how mass shootings and mass killings are defined to identify and avoid potential bias in addressing the topic.

Mass Killing Definition

A mass killing is legally defined as “three or more killings” during an incident, again excluding the death of the perpetrator, per a mandate by President Barack Obama in the Investigative Assistance for Violent Crimes Act of 2012.

In this legal definition, a mass killing is qualified strictly by the amount of fatalities, again not taking the killer’s context, relationship, or location into account. It also does not dictate the manner of death, which serves as a reminder that not every mass killing is a mass shooting. The opposite holds true as well: a mass shooting, if less than four people are killed, will not be considered a mass killing. 

Mass Shooting Statistics

At time of writing, according to the Gun Violence Archive, there have been 620 mass shootings in the United States in 2022. This number includes every shooting incident in which four or more people were injured or killed regardless of motivation or location. Of those 620 mass shootings, 36 of themroughly 6%qualify as mass killings, incidents in which four or more people (excluding the shooter) are fatally shot. 

As a point of comparison, 2021 ended with 690 mass shootings, 2020 saw 611 and 2019 had 417. Below are more statistics about the state of mass shootings in the United States and beyond.

Where Do Most Mass Shootings Happen?

The highest majority of mass shootings happen in the workplace, accounting for 33% of all mass shootings in a research study looking at active shooter events from 1966 to 2019.

The next most common places were retail establishments and bars/restaurants, with 16.9% and 13.4% respectively. Fourth and fifth on the list are residential homes and outdoor locations, both amounting to 8.1% of the total.

Schools (7.6%), places of worship (6.4%), and universities (5.2%)  all account for smaller percentages, though these locations tend to account for some of the more horrific, high-profile tragedies.


School Shootings Statistics

According to the Washington Post, 346 schools have been affected by school shootings since the Columbine High massacre in 1999, leaving 190 children and educators killed with another 404 sustaining injuries. 2021 was the worst year for school shootings, with 42 gun-related attacks occurring. There have been 24 similar attacks reported so far in 2022.

The Washington Post also runs a comprehensive database of school shootings and estimates that 323,000 students have been impacted by mass shootings. This number comes into focus as we consider not just the wounded and dead after an active shooter incident, but those left scarred for life by the memory of something unthinkable happening to them in a place they felt safe. The fear of these tragedies is also growing, with the rise of school shootings being blamed for nationwide teacher shortages

Learn more about education security in the fight against school shootings. 

Workplace Shooting Statistics

Though the shocking statistics around school shootings seem to attract more media, the truth is workplace shootings are far more frequent. Since 1999, there have been four times as many shootings in the workplace as there has in schools. But just as with school shootings, 2021 also proved to be the most deadly year on record for workplace shootings as well, resulting in a 50% year-over-year increase in requests for mass shooter insurance

Worth noting is that 45.6% of workplace shootings took place in retail locations, with the vast majority (68.2%) of shooters not being employed by the business under attack.

Less than half of businesses have a plan for a crisis in the workplace, yet according to AlertMedia, 53% of employees are “very concerned” about workplace violence, with 90% in agreement that employers have a moral and legal obligation to protect them from unnecessary risk while working.

Learn more about commercial security solutions that can help prevent a mass shooting in your workplace. 

Government Facility Shootings

Shootings in a government facility exist as a subset of workplace shootings, but are no less significant, accounting for 1 in 4 shootings that occur at work since 2006. The 2021 US Capitol breach showed that even highly secured federal buildings are at risk of catastrophic emergencies, and need added layers of protection.

Learn more about security solutions for government grounds where maximum safety is required.

Gun-Related Deaths in 2021

According to a study by the U.S. Centers for Disease Control and Prevention (CDC) 2021 was the most deadly year for gun-related violence in the America in the last 30 years. Almost 21,000 people died from gun-related homicide in 2021, which is an 8% increase from the previous year

Though this reflects an overall rise in homicide from the year before, firearms are playing a larger role in these deaths. In 2020, 79% of homicides were gun related, compared to 81% in 2021. These trends are in agreement with our findings about 2021 being a peak year for both school and work shootings. 

When asked to hypothesize why 2021 proved to be an especially deadly year for our nation, Harvard professor David Hemenway suggested the COVID-19 pandemic may have played a role. Gun sales rose during that uncertain time, with nearly 18% of Americans purchasing a gun between March 2020 to March 2020, with 5% of them being first-time gun buyers

Though thankfully 2022 isn’t proving to be as deadly, the upward trend in gun-related violence can’t be denied.

What Weapons Are Used In Mass Shootings?

Though semi-automatic firearms were used in the five deadliest mass shootings from 2009-2020 (Las Vegas, Orlando, Newtown, Sutherland Springs, and El Paso) and attract the most media attention, they are not the most common weapons used in mass shootings. 

  • Handguns
    77% of mass shootings involved the use of at least one handgun (some used multiple handguns, while others were combined with assault rifle use). These easily concealed weapons are designed for use by one hand, especially a pistol or revolver.
  • High-Capacity Magazines
    High capacity magazines allow an active shooter to fire more rounds before having to reload their weapon. With the ability to fire more rounds consecutively, they are able to inflict more injuries during an attack. 55% of mass shootings involve a high-capacity magazine.

Do Mass Shootings Happen Outside the United States?

Mass shootings occur in virtually every country around the world. However, just as we have difficulty defining the term in our own country, we face further trouble as we attempt to extend the scope of these statistics. As we look beyond the United States’ statistics, and attempt to make a comparison with other countries, we run into the following troubles:  

  • Available Data
    Thanks to nonprofits like the Gun Violence Archive we have access to reliable data on mass shootings across the US. Not every country has such a resource, and fewer still are as transparent in disclosing this information as United States law enforcement is.


  • Battles Over Sovereignty
    Even if there were an agreed-upon metric for what defines a mass shooting, there are situations that remain questionable as one looks at data of mass shootings outside the United States. Researchers often omit mass shootings they characterize as “battles over sovereignty” in countries with contested borders. Situations like this are not found in the United States and have proven hard to categorize abroad.


  • Regional Violence
    Countries with state-sponsored violence, known drug trafficking operations, and increased terrorist activity may see heightened levels of mass shootings that go unreported or would otherwise pose difficulties when qualifying them for comparative statistics.


Despite the difficulties in reporting, two facts emerge from the data available to us. Yes, mass shootings happen outside of the United States, however, they occur here in a disproportionate amount. According to a study by Adam Lankford in the Econ Journal Watch, the United States is responsible for 28.3% of mass shootings worldwide despite being home to only 4.5% of the world’s population. 


Has There Been A Mass Shooting Today?

The Gun Violence Archive (GVA) is a reliable source for non-biased data on gun-related violence, with up-to-date information about all mass shootings defined as four or more victims reported in the United States. Cited in a recent Supreme Court Case about gun-related violence, this trusted nonprofit website presents clear, vetted statistics in a variety of formats, including charts and maps, that can lend perspective to the current trends in mass shootings. 

For more information about the state of mass shootings in America, we encourage you to subscribe to the ZeroEyes Newsletter. Our monthly updates provide you with timely information on recent mass shooting statistics and resources on how you can help prevent gun-related violence. 

How to Prevent Mass Shootings

The statistics and biases we explored above can help guide our thinking about mass shootings,  but being aware of the problem doesn’t necessarily bring us any closer to solving it. 

Despite better data and technology than ever before in human history, mass shootings are on the rise today, and the world is in urgent need of proactive solutions that can be implemented immediately. We’ve struggled for decades at ways to make our country safer, but the statistics show that old ways of thinking are not going to solve the crisis of mass shootings. 

Thankfully, a new solution is proving effective in the security space that can help prevent mass shootings: visual A.I. gun detection technology. ZeroEyes’ proprietary software integrates with existing security cameras and alerts a military-trained analyst in our operations center as soon as a gun is detected on your property. From there, our team verifies and dispatches the threat as fast as 3-5 seconds to:

  • Improve Police Response Times
    By identifying a firearm by first sight, not first shot, ZeroEyes puts you at an advantage against the shooter. As soon as our 24/7/365 team confirms the A.I.’s gun detection assessment, they’re immediately on the phone with local authorities, before panic and confusion set in at the scene. In a crisis, mere seconds are critical.


  • Actionable Intel
    In many active shooter scenarios, first responders arrive at the scene with uncertainty about where to go or who to look for. When ZeroEyes dispatches a gun detection, we provide law enforcement with the exact geolocation of the shooter, a visual photograph, and firearm identification.

    As the shooter moves through a facility and passes by cameras supercharged with ZeroEyes’ A.I., we can also provide law enforcement with a breadcrumb trail that clearly shows the shooter’s most recent location. This trail can be used to: A) Locate and stop the perpetrator faster B) Effectively triage by sending aid to probable locations where victims need immediate care.


Visual A.I. gun detection can help save lives by alerting authorities of an illegally brandished gun before shots are fired, improving police response and providing clarity during a time of chaos. Contact us to learn more about how ZeroEyes can help you prevent the next mass shooting and lower the statistics discussed here today. 


Rob Huberty
Rob Huberty is the Chief Operating Officer & Co-Founder of ZeroEyes. Before founding ZeroEyes in 2018, Mr. Huberty worked as a Pathways Operations Manager at Amazon, leading cross-functional teams, initiating process improvement, developing standard operating procedures, and serving as a mentor for his teammates. He served in the U.S. Navy as a Navy SEAL where he collaborated with international governments to conduct special operations, trained foreign forces to ensure mission success, and employed innovative problem solving.

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