How many mass shootings have there really been in 2017? It depends on the source

A Las Vegas police officer stands guard as traffic continues to be blocked on the Las Vegas Strip after a lone gunman opened fired on the Route 91 Harvest country music festival on October 2, 2017 in Las Vegas, Nevada. The gunman, identified as Stephen Paddock, 64, of Mesquite, Nevada, allegedly opened fire from a room on the 32nd floor of the Mandalay Bay Resort and Casino on the music festival, leaving at least 58 people dead and over 500 injured. According to reports, Paddock killed himself at the scene. less A Las Vegas police officer stands guard as traffic continues to be blocked on the Las Vegas Strip after a lone gunman opened fired on the Route 91 Harvest country music festival on October 2, 2017 in Las Vegas, ... more Photo: David Becker/Getty Images Photo: David Becker/Getty Images Image 1 of / 10 Caption Close How many mass shootings have there really been in 2017? It depends on the source 1 / 10 Back to Gallery

On Monday morning, many in social media and the news reported that the Las Vegas tragedy is the United States' 273rd mass shooting of 2017.

The factoid was widespread throughout Twitter, mentioned on the BBC's live feed and the focus of a Newsweek article titled "There's a mass shooting almost every day in the U.S." stating, "The horrifying events in Las Vegas on Sunday night mark the 273rd mass shooting in the U.S. in the 275 days that have passed so far in 2017, according to Gun Violence Archive."

Gun Violence Archive is one source that tracks "mass shootings" with an online database. Mother Jones is another. The progressive American magazine updated its database Monday morning and its count for 2017 now stands at seven.

Why the vast discrepancy?

There's really no one definition of mass shooting.

The nonprofit, nonpartisan group Gun Violence Archive defines mass shooting as "four or more shot and/or killed in a single event [incident], at the same general time and location not including the shooter." The Mass Shooting Tracker, a crowd-sourced list that's also often cited in social media, uses the same definition.

In other words, if an attacker kills three people with gunfire and no one else is injured, it doesn't count as a mass shooting by the Gun Violence Archive definition. If an attacker shoots and injures four people, the bullets only grazing them, and nobody dies, it counts.

Mother Jones, on the other hand, defines "mass shootings" as "indiscriminate rampages in public places resulting in four or more victims killed by the attacker." This narrower definition results in the significantly lower total count for 2017.

Even within the U.S. government, the definition seems to vary.

A 2013 report on Public Mass Shootings in the United States says that "there is no broadly agreed-to, specific conceptualization" of mass shootings. For the report, researchers used the definition: "These are incidents occurring in relatively public places, involving four or more deaths—not including the shooter(s)—and gunmen who select victims somewhat indiscriminately. The violence in these cases is not a means to an end—the gunmen do not pursue criminal profit or kill in the name of terrorist ideologies, for example."

SFGATE reached out to the FBI for their definition and spokesperson Andrew Ames wrote in an email, "our definitions are based on federal law" and directed us to the bureau's Active Shooter Resources site that uses a broad definition to define mass killings as three or more people killed regardless of weapon.

The lesson in all of this is that when you see counts for mass shootings in your social media feeds, it's important to consider the source's definition and understand that not all sources are neutral in their politics.

James Alan Fox, a professor of criminology at Northeastern University in Boston, is concerned about those sources that use a broader definition.

"These stats where four or more people were shot... most of those aren't mass killings and in a high percentage of those, no one dies," Fox says. "They roll out those numbers every time there's a mass killing which confuses people. They start equating that things like Las Vegas happen every day. And they don't happen every day. It's really a disservice to try and broaden the definition."

Fox also feels Mother Jones' numbers are problematic because, "they ruled out all family executions, which are half the cases. They also ruled out cases that are robberies. Only the public cases are included and that's usually about a half-dozen a year."

He adds that it can be difficult to define whether a shooting occurred in a public place and says one of the incidents Mother Jones includes in its database occurred at a private party, yet they don't cite any other home incidents.

He thinks the most accurate definition is four or more victims, not including the killer, and encompasses family, public and burglary killings, and this is the one the U.S. Congressional Research Service uses. With this definition, he says on average there are about two dozen mass shootings a year.