If it seems like mass shootings are becoming more common, researchers say there's a good reason: They are.
Between a 2011 shooting at an IHOP restaurant in Carson City, Nev., that left four people dead and the 2013 attack on the Washington Navy Yard where 12 people were killed, a mass shooting occurred somewhere in America once every 64 days, on average.
In the preceding 29 years, such shootings occurred on average every 200 days, according to an analysis by researchers from Harvard University's School of Public Health and Northeastern University.
The study defined a mass shooting as an outbreak of firearms violence in which four or more victims were killed and the shooter was unknown to most of his victims.
Not only are such shootings more common, they have also become more deadly. In the 10-year period that ended with the Washington Navy Yard attack, a total of 285 people died in such events. In the 13 years before that, 151 people perished in mass shootings.
Between Jan. 1, 2014, and May 26, 2015, 195 more people in the United States have been slain in an additional 43 shootings, according to statistics drawn from Mass Shootings Tracker, a Wiki-style site.
That doesn't include the nine victims killed Wednesday night at the Emanuel African Methodist Episcopal Church in Charleston, S.C.The per capita number is misleading, since the number of American households without guns has increased over the decades, meaning that more guns are concentrated in the fewer households which have them.
Although the fatalities in mass shootings are dramatic, they are dwarfed by the number of people killed by firearms in attacks that affect one or two victims at a time and largely escape public notice. The Centers for Disease Control and Prevention estimated that 11,208 people died in homicides involving firearms in the United States in 2013.
Today, American civilians are thought to own as many as 310 million firearms, according to the Bureau of Alcohol, Tobacco and Firearms. A 2012 report from the Congressional Research Service noted that the number of guns per capita had doubled since 1968.