Every time there’s a shooting in the United States that leaves multiple people injured or dead, a few things happen in reaction. Public officials express their condolences, there’s often a call for legislative action. And reporting begins to come out referencing how many similar shootings have taken place that year.

The problem is, you might see a few different numbers, all trying to quantify the same thing: the number of “mass shootings” during a given time.

And they’re often radically different numbers.

This inconsistency is at the heart of a lot of debate online and in communities across the country. And it’s the result of a couple of problems.

No. 1: There Isn’t Agreement On What Makes Something A ‘Mass Shooting’

Unfortunately, everyone seems to define mass shootings a little differently. Traditionally, it’s been a combination of two factors: how many people were injured or killed, and the type of violence being committed.

The first is rather straightforward.

The second, on the other hand, can be challenging to define in the immediate aftermath of an event. We often don’t know if an incident was an act of domestic or gang violence, terrorism, or an indiscriminate act of violence until days or weeks later.

Let’s start with one of the more generous definitions, used by the Gun Violence Archive, a nonprofit that has been tracking “gun-related violence” in the U.S. since 2013. The Gun Violence Archive is updated daily, and as a result, is often cited when people are looking for “the latest” numbers on gun violence.

They also have one of the most expansive definitions of mass shootings. They include every incident where “four or more [are] shot or killed, not including the shooter.”

The Gun Violence Archive does not remove any type of shooting from its counting. This means gang or domestic violence is included and does not “exclude, set apart, caveat, or differentiate victims based upon the circumstances in which they were shot.” This consistently leads to them having a much higher total number of mass shootings than other tallies. It also means that incidents that might not be included in other lists appear in their tally.

Another commonly referenced resource is the FBI. They provide detailed annual reports on incidents, but they usually aren’t available until the new year.

The FBI does not use the term “mass shooting” and instead refers to these events as active shooter incidents. They exclude drug or gang violence and “accidental discharges of a gun.”It says an “active shooter is an individual actively engaged in killing or attempting to kill people in a populated area.”

Meanwhile, in 2013, Congress defined mass killing as “[three] or more killings in a single incident,” as part of a bill giving the Department of Justice the ability to support local authorities during an investigation. This definition creates confusion by introducing a new minimum number of victims and increases the number of incidents included in a Congressional tally.

For our part, unless otherwise stated, Guns & America defines these incidents as the shooting of two or more people in a single incident, in a public place. This definition excludes crimes of armed robbery, gang violence, or domestic violence, focusing on cases in which the motive appears to be indiscriminate mass murder.

One of the places you’ll hear or see “mass shooting” used the most is by the media. Most organizations have style guides they follow that say when certain language should — or should not — be used.

The Associated Press, which maintains one of the most widely used instances of these guides, doesn’t provide a definition. It instead suggests the phrase “mass shooting” should be avoided.

Mother Jones maintains one of the most cited databases of mass shootings (more on that below) and uses its own definition, which combines much of what we’ve already outlined: “The killing of four or more people in a single incident, in a public place.” It excludes “crimes of armed robbery, gang violence, or domestic violence in a home, focusing on cases in which the motive appears to be indiscriminate mass murder.”

And so, for example, the Gun Violence Archive has a record of 340 mass shootings in 2018, including a November 2018 drive-by shooting in Oakland, California, where four people were injured. That incident is not listed in Mother Jones’ tally for 2018, or in the FBI report on active shooter incidents in the U.S. in 2018. Mother Jones lists 12 mass shootings in 2018 while the FBI’s report includes 27 incidents last year.

No. 2: Our Understanding Of These Incidents Has Evolved

To make it even more confusing, as we’ve learned more about these type of events, our definitions have changed. So what might have fit the definition at one point may not quite meet the criteria now.

Newer definitions are broader and leave room for a more subjective interpretation, as with the FBI’s definition of an active shooter incident. In the past there’d been mention of the number of people injured or killed (a minimum of four) “it used to define as “in one event, in one location” but now there’s no longer emphasis of the minimum number of individuals killed or a single location. That’s all gone.

And this inconsistency is not helpful when you’re trying to understand a situation where people are being hurt and information is evolving.

No. 3: Mass Shootings Represent A Sliver Of Gun Violence In America

According to the latest CDC data from 2016, active shooter incidents represent less than 2% of all gun deaths annually, compared with suicides (about 60%) and homicides (about 30%). This makes them incredibly hard for researchers to study because the pool of data is so small compared to gun violence overall in the U.S.

So What Now?

So how do you count mass shootings? It depends on what you care about. Often we start with fatalities and injuries and then motivation, circumstances, or other factors. When you see mention of the number of mass shootings, it’s important to contextualize the numbers in relation to the source of the data.

Each resource that tracks these incidents abides by their own definition, which they should make available on their site or along with their reporting.

If another news organization is citing a stat, pay attention to where it’s coming from and which types of gun violence are included or excluded. Certain resources have pros and cons (more on that below) and should be used accordingly in reporting and in understanding the news you read.

In discussions or when sharing information, be mindful of the source. It’s often easy for a tweet or a Facebook post to exclude or drop the citation. Without the context each definition provides, these numbers lose their meaning.

Sources That Track Gun Violence

Mother Jones

Mother Jones’ database covers mass shootings in the United States from 1982 to present and includes data on mental health issues, what weapon was used and how it was obtained and sources.

Pros: It’s updated quickly and the database is downloadable if you want to explore the numbers yourself.

Cons: What is and is not included can sometimes feel subjective so it’s important to read the accompanying guide before digging too deep or if you’re reading a story that relies on numbers from Mother Jones’ database.

The Gun Violence Archive

This archive includes information on gun violence nationally, defensive use, officer-involved shootings in addition to mass shootings going back to 2014.

Pros: It’s updated quickly and has great interactive visualizations of the data.

Cons: Its inclusive definition for mass shooting can result in a higher number than other sources, making it important to cite when sharing their work.

The FBI

The FBI releases annual reports on active shooter incidents, with state-by-state details, details on shooter behavior and conclusions on trends. They also release occasional reports on the subject that are incredibly informative.

Pros: They’re a terrific reliable resource.

Cons: Reports are annual at best, sporadic at worst, so if you’re looking for the latest numbers, other resources are better.

The Secret Service

The Secret Service’s National Threat Assessment Center released its second report this year as part of an effort to prevent targeted violence in the U.S. The report covers what it calls “targeted violence” or a “mass attack” or “in which three or more persons were harmed, were carried out in public spaces within the United States” This is similar to the Congressional definition of “mass killing.”

Pros: The report goes into great detail on what we know about the perpetrators.

Cons: Another annual report, with another distinct definition.

Lisa Dunn and A.C. Valdez contributed to this reporting.

Guns & America is a public media reporting project on the role of guns in American life.