Mass shootings: No one's sure how many

Jabin Botsford | The Washington Post | Getty Images

People examine handguns during The Nation's Gun Show in Chantilly, Virginia, on Oct. 3, 2015.

San Bernardino is not the first mass shooting of 2015, but there's little consensus on how many there have been.

It turns out there's no standardized way of measuring mass shootings in the U.S. Some methods say the number of mass shootings is reaching unprecedented levels, while some say the number has actually dropped in recent years.

Almost as quickly as news spread about the shooting in California, news outlets around the country were declaring this the year of the mass shooter. "TODAY" wrote in a headline that there have been "355 mass shootings in 336 days: US has more incidents than days in 2015." On the other hand, Mother Jones, which maintains its own tracker of mass shootings, claims there have been 72 since 1982.

Different methods of tabulation, as well as different measures of "mass" attacks lead to wildly divergent data. With so much confusion, it can be easy for the public to feel overwhelmed by gun violence and for politicians and interest groups to find data that fit their preferred narrative.

There are many reasons for the vastly different figures. For one thing, different agencies and media outlets collect information about shootings differently and define "mass" in wildly different ways.

The FBI, for example, defines "mass murder" as the killing of four or more individuals "occurring during the same incident, with no distinctive time period between the murders." The most recent data available from the FBI are from 2013, and suggests mass murders have occurred in the low double digits through the past few years.

In response to the FBI's data aggregation, two Redditors in 2013 started the Mass Shooting Tracker. The tracker defines "mass shooting" as four or more people shot in one event, not just those murdered. The rationale is that a shooting that involves four or more victims should be treated with gravity no matter how many are killed and that the gun lobby minimizes gun violence by focusing on deaths rather than victims.

According to the Mass Shooting Tracker, there have been 353 mass shootings so far in 2015, more than in all of 2014.

The Gun Violence Archive, a nonprofit also founded in 2013, counts 310 mass shootings so far in 2015. The organization defines "mass shooting" as "four or more people shot and/or killed" in a single incident, not including the shooter(s) themselves.

The Stanford Mass Shootings in America project, a data aggregation effort founded in 2012 in the wake of the Sandy Hook shootings, defined "mass shooting" as "three or more shooting victims (not necessarily fatalities), not including the shooter." The Stanford database also weeds out shootings that are "identifiably gang- or drug-related" to focus on "indiscriminate killing."

Stanford's database shows a much lower number of mass shootings over the past 10 years than many other trackers, but a huge increase in 2015.

USA Today's "Behind the Bloodshed" database uses the FBI's definition for mass killings.

The paper includes killings that occur over a day or more and distinguishes mass killings from serial killings by an extended "cooling-off period." In addition to shooting victims, it includes those killed by stabbing, blunt force and smoke inhalation. (We subset the shooting category for the chart above.)

The progressive Mother Jones uses one of the more conservative methodologies for its mass shooting tracker. They also use the FBI's definition of mass murderer (four or more fatalities), with the added addendum that "the killings were carried out by a lone shooter." (They make exceptions for the Columbine massacre in 1999 and the Westside Middle School shootings in 1998.)

Mother Jones also includes a "handful of cases" of spree killings, those mass murders that take place in more than one location, but over a short period of time. Mother Jones has an explainer of its methodology on its website.

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One reason for the lack of consistency among methods for counting mass shootings could be the effective ban on research around gun violence imposed on the Centers for Disease Control and Prevention.

In 1996, Rep. Jay DIckey, an Arkansas Republican, worked to defund the CDC of millions of dollars and added a stipulation that "none of the funds made available for injury prevention and control at the Centers for Disease Control and Prevention may be used to advocate or promote gun control."

In 2012, Dickey co-authored an opinion piece in the Washington Post with Mark Rosenberg, former director of the CDC, in which they called for research and the same "evidence-based approach" to gun violence that has saved millions of lives from car accidents and smoking.