But not everyone agrees on how to define a mass shooting.

Such figures are cited in the debate over what to do to curb gun violence in this country.

There has been growing interest in recent years in tracking the number of “mass shootings” in the United States.

The tally of such incidents can vary dramatically, depending on the definition.

The following graphic shows some of the more commonly cited sources for mass shooting data, and the total number of incidents each tracked during 2016.

How many mass shootings were there in 2016? Mass shootings (according to the Mass Shooting Tracker) 476 Mass shootings (according to The Gun Violence Archive) 385 Mass shootings (according to USA Today) 22 Mass public shootings (according to USA Today) 5 Mass shootings (according to Mother Jones) 6

For more information on the definitions used and the groups involved — and to get a sense of annual trends, check out the following:


Mass Shooting Tracker

• The definition: A single incident in which four or more people were shot, regardless of whether they were killed. The definition allows for the shooter to count toward the four or more threshold. [More on definition, methodology here]

• Who tracks it: The data is crowdsourced by members of the GunsAreCool subreddit, which is a pro-gun control forum on the website Reddit.

Gun Violence Archive

• The definition: A single incident in which four or more people were shot at the same general time and location, regardless of whether they were killed. The shooter does not count toward the four or more threshold. [More on definition, methodology here]

• Who tracks it: The Gun Violence Archive describes itself as a non-profit formed in 2013 to track data on gun violence.

USA Today (mass shootings)

The definition: A single incident in which four or more people, excluding the shooter, were shot to death. The incident can stretch over a day or more and “some distance,” but it cannot include an “extended ‘cooling-off period’” between killings “to distinguish this kind of crime from the acts of serial killers.” [More on definition, methodology here]


• Who tracks it: National news outlet USA Today.

USA Today (public mass shootings)

• The definition: A single incident in which four or more people, excluding the shooter, were shot to death in a public setting. The incident can stretch over a day or more and “some distance,” but it cannot include an “extended ‘cooling-off period’” between killings “to distinguish this kind of crime from the acts of serial killers.” [More on definition, methodology here]

• Who tracks it: National news outlet USA Today.

Mother Jones

• The definition: A single incident in which four or more people were shot to death in a public place in which the motive appeared to be indiscriminate killing. The definition typically has not counted the shooter toward the four or more threshold, but at least one exception has been made. The definition excludes shootings tied to gang activity, robberies, domestic violence, and other crimes. [More on definition, methodology here]

• Who tracks it: Investigative news outlet Mother Jones.

Other sources of historical data on mass shootings, active shooter incidents

• The Congressional Research Service in 2015 released a report detailing mass shooting incidents between 1999 and 2013. The report defined a mass shooting as a single incident in which four or more people, excluding the shooter, were shot to death in close geographical proximity. [More on definition, methodology here]

• The Congressional Research Service’s 2015 report also broke the data down further to look at mass public shootings, which were defined as: a single incident in which four or more people, excluding the shooter, were shot to death, within a close geographical proximity and with at least some of the murders occurring in a public location and unrelated to any other underlying criminal activity, such as armed robberies, arguments, romantic triangles, insurance fraud, or criminal competition. [More on definition, methodology here]


• The FBI in 2014 released a report on “active shooter” incidents between 2000 and 2013, and this year released a follow-up report detailing incidents in 2014 and 2015. The reports defined an active shooter incident a single incident involving one or more people “actively engaged in killing or attempting to kill people in a populated area.” [More on definition, methodology here]

Matt Rocheleau can be reached at matthew.rocheleau@globe.com. Follow him on Twitter @mrochele