“The ‘incidence rate’ has quadrupled since the 1970s. If it seems like mass shootings in public places have become more commonplace in the United States after a summer of deadly shootings in Charleston, Chattanooga and elsewhere, that’s because they are. According to a recent study released by the Congressional Research Service, the average rate of mass public shootings—defined by the FBI as ‘a multiple homicide incident in which four or more victims are murdered with firearms, within one event, in at least one or more public locations’—has gone up from being a singular, annual tragedy in the 1970s (a 1.1 incidence rate) to about 4.5 incidences per year between 2010 and 2013.”
Source: This Chart Shows How Mass Public Shootings in the U.S. Have Risen | TIME