U.S. Mass Shootings: Just the Facts

The number of killed and injured victims from shootings in the United States since 1960.

The number of killed and injured victims from shootings in the United States since 1960.

Graphics and text by ROGER WARBURTON/ecoRI News contributor

In the past decade, the number of victims from shootings in the United States has increased substantially. This increase has occurred despite a 50 percent decrease in firearm homicides since 1993. (The decrease in firearm homicides is generally attributed to better policing and a better economy.)

These incidents are often referred to as “mass shootings,” which are defined here as an incident in which a shooter kills at least four people in one location at about the same time. The count typically excludes the perpetrators, gang killings, domestic violence, and terrorist acts.

Some studies indicate that the rate at which mass shootings occur has tripled since 2011. Between 1982 and 2011, a mass shooting occurred roughly once every 200 days. However, between 2011 and 2014, that rate accelerated significantly, with at least one mass shooting occurring every 64 days.

This table shows that the accelerating trend, begun in the 1960s, continues through the present day.

This table shows that the accelerating trend, begun in the 1960s, continues through the present day.

It’s also interesting to note that the number of mass shootings began to rise significantly in the 1960s. Until 1960, the number of incidents and victims is small, but the rate begins to rise rapidly through the decade.

The large peak in 2017, shown in top graphic, is dominated by data from the Oct. 1 shooting of that year in Las Vegas, in which a man in a high floor of a hotel opened fire on a country music festival, killing 58 people and injuring 851 others, with 422 of them suffering from gunshot wounds. The man then shot himself.

One might consider that the enormous number of 422 people wounded during the Las Vegas shooting is an outlier that distorts the chart scale and dominates the data. Therefore, in the graphic below, we present the data without the large number of wounded in that shooting.

Mass shootings in the United States from 1960 to 2019, excluding the Las Vegas shooting from Oct. 1, 2017.

Mass shootings in the United States from 1960 to 2019, excluding the Las Vegas shooting from Oct. 1, 2017.

This graphic shows, even more dramatically, the accelerating and relentless rise in the number of victims from mass shootings. From a technical, data analysis perspective, this graphic shows how the removal of outliers often improves the chart and clarifies the message to be communicated.

In general, however, it’s not acceptable to drop an observation just because it’s an outlier. Here, since we aren’t performing a statistical analysis, the Las Vegas shooting doesn’t change the results or affect the assumptions.

In fact, the Las Vegas data point is a legitimate observation and one of the most interesting data points. It’s up the reader to decide its importance. It’s also worth pointing out that there is no standard or agreed definition for what constitutes a mass shooting.

One generally accepted definition is that a mass shooting is an incident involving several victims of firearm-related violence, but the precise inclusion criteria are disputed, and there is no broadly accepted definition.

The Gun Violence Archive, a nonprofit research group that tracks shootings and their characteristics in the United States, defines a mass shooting as an incident in which four or more people, excluding any perpetrators, are shot in one location at roughly the same time.

The Congressional Research Service uses a much narrower definition that only includes victims who are killed and excludes any victims who survive. The Washington Post and Mother Jones use similar definitions, with the latter acknowledging that its definition “is a conservative measure of the problem.”

The crowdsourced Mass Shooting Tracker project uses a looser definition: four people shot in one incident regardless of the circumstances.

The Investigative Assistance for Violent Crimes Act of 2012, signed into law in January 2013, defines a “mass killing” as one resulting in at least three victims, excluding the perpetrator. In 2015, the Congressional Research Service, in a report entitled Mass Murder with Firearms, defined a mass hooting as “a multiple homicide incident in which four or more victims are murdered with firearms, within one event, and in one or more locations in close proximity.”

Roger Warburton, Ph.D., is a Newport, R.I., resident.

Editor’s note: Data used in the graphics were from the Wikipedia page list of mass shootings in the United States, which was updated to include the event in Midland-Odessa, Texas, on Aug. 31, when six people were killed and 21 others injured.