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The frequency of high-fatality mass shootings has increased significantly

Shooting incidents killed at least 10 every four years from 1984 to 2004. There have been four this year.

Police officers close off an area near the intersection of US 101 freeway and the Moorpark Road exit in response to a shooting at a bar in Thousand Oaks, Calif., on Nov. 8, 2018. (FREDERIC J. BROWN/AFP/Getty Images) (Afp Contributor#afp)

A shooter at a bar in Thousand Oaks, Calif., early Thursday murdered 12 people, including a sheriff’s deputy responding to the incident. It’s the worst shooting incident in the United States in ... a bit over a week.

There’s an ongoing debate about whether shooting incidents generally are becoming more frequent. The rate of violent crime in the United States has dropped significantly since the late 1980s and early 1990s and, despite some political rhetoric, remains near all-time lows. But there’s little question that the frequency of high-fatality mass shootings -- for our purposes, incidents in which at least 10 people are killed -- has increased.

Mother Jones magazine has tracked mass shootings since 1982, including any incident in which at least four people were killed. Its database includes information on the shooting, such as the shooter’s demographic information, apparent motive and weapon of choice.

Below is a graph of all of the incidents that match Thousand Oaks, with at least 10 people killed by the shooter. The horizontal bars show the number of days since the last such incident. The longer the line, the more time had passed.

From 1984 to 2004, there was an incident in which at least 10 people were killed about once every four years. Over the past four years, there have been eight. There have been four such incidents this year alone.

The shooter in Thousand Oaks apparently used a handgun, uncommon for recent shootings. Since the mass shooting at a movie theater in Aurora, Colo., in 2012, most of the incidents in which at least 10 people were killed have included the use of deadlier weapons, such as a semiautomatic rifle (eight of 10 incidents) or a shotgun (four of 10 incidents), often in addition to a handgun (7 of 10 incidents).

Since the assault weapons ban expired in 2004, there have been 14 incidents in which at least 10 people have been killed, eight of them involving the use of a semiautomatic rifle. Three prior to Thousand Oaks involved only a semiautomatic handgun, according to Mother Jones’s data.