What’s a Mass Shooting? Statistics Difficult to Define

There’s no question that one mass shooting is one too many, and the horrific attack in San Bernardino earlier this month only reinforced that sentiment. The event has left a community and families in tears, and lawmakers grappling with some very difficult questions.

Since the shooting at the holiday party at the Inland Regional Center, a variety of mass shooting stats have populated newspaper headlines, and radio and television airwaves, stoking fears that these types of incidents are on the rise in the United States.

But experts told NBC Bay Area the mass shooting numbers aren’t actually that telling.

"I think there’s some evidence that indiscriminate killings have gone up," said Robert Weisberg, co-director of Stanford’s Criminal Justice Center. "But I don’t think they represent a huge percentage of American deaths. It doesn’t show that Americans are more vulnerable to being killed in murders than they were twenty years ago, because that’s simply not true."

After all, determining any trend in mass shooting statistics depends largely on how one defines the term mass shooting.

The FBI studied the history of active shooters, which the agency defines as anyone "actively engaged in killing or attempting to kill people in a populated area," from 2000 to 2013. Over that time span, 160 incidents were recorded. The first seven years saw 45 incidents. In the second half of that time span the number jumped 250 percent to 115 incidents.

The Shooting Tracker survey courtesy of Shootingtracker.com, a Reddit group that has crowd-sourced data since 2013, utilizes a broad definition of mass shooting. The survey includes gang and domestic violence, and no minimum number of deaths. According to that source, there have been more mass shootings in 2015 thus far than there are days in the year.

Mother Jones magazine might have the most discriminating criteria for "mass shooting" data. The publication only counts shootings that take place in a public setting, where the primary motive is mass murder. The magazine found 38 incidents in the past decade, more than in the two previous decades combined.

Despite the appearance of a rising trend in these numbers, it’s important to note that even using the loosest definition of mass shooting, these deaths account for 1.5 percent of all firearm homicides in the United States.

And the gun murder rate in the country has been declining for decades.

Today’s mass shooting conversations also leave out a critical element, according to Stanford’s Robert Weisberg.

"The increase in the number of killings in that category is still not that large relative to population growth," he said. "In fact, it may...wash out, which means once again it’s a matter of perception of significance."

All that being said, make no mistake: Compared to the rest of the world, the United States is still in a league of its own for gun violence.

The United States still has more gun-related deaths per 100,000 people than any other developed country on earth.

"I talk to my foreign law-enforcement counterparts, and you know, they don't understand how something like this could happen on a fairly frequent basis in the United States," said the FBI’s San Francisco Special Agent in Charge David J. Johnson. "It's no pun intended, but it's foreign to them." 

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