The most-used definition of a "mass shooting" is that four or more people are killed in a single incident. Here's the pattern from 1999-2013.
Mass shootings can be divided into three categories: mass public shootings, "familicide" shootings that involve a group of family members, and felony mass shootings that would include gang executions, botched robberies and hold-ups,and the like. The chart above includes all three of these. The breakdown in the three categories is below. Mass public shootings have about half of the incidents of each of the other two categories, but include almost as many victims killed and more victims injured.
What if we just look at the pattern for mass public shootings over time, leaving out the familicides and other felony mass shootings? Here's the pattern. If you squint a bit at 2007, 2009 and 2012, you can sort of imagine an upward trend here, but given that there's a lot of annual fluctuation, it's not clear that the trend is a meaningful one over this time frame.
However, if one takes a longer time-frame going back several decades, it does appear that mass public shootings have risen. Krouse and Richardson write:
With data provided by criminologist Grant Duwe, CRS [the Congressional Research Service] also compiled a 44-year (1970-2013) dataset of firearms-related mass murders that could arguably be characterized as “mass public shootings.” These data show that there were on average:
• one (1.1) incident per year during the 1970s (5.5 victims murdered, 2.0 wounded per incident),
• nearly three (2.7) incidents per year during the 1980s (6.1 victims murdered, 5.3 wounded per incident),
• four (4.0) incidents per year during the 1990s (5.6 victims murdered, 5.5 wounded per incident),
• four (4.1) incidents per year during the 2000s (6.4 victims murdered, 4.0 wounded per incident), and
• four (4.5) incidents per year from 2010 through 2013 (7.4 victims murdered, 6.3
wounded per incident).
These decade-long averages suggest that the prevalence, if not the deadliness, of “mass public shootings” increased in the 1970s and 1980s, and continued to increase, but not as steeply, during the 1990s, 2000s, and first four years of the 2010s.
As another way of illustrating this longer-term upward pattern of mass public shootings, consider only mass public shootings in which 10 or more people were killed. Up through 2013, there had been 13 such episodes in modern US history: seven occurring from 2007-2013, and six during the 34 years from 1966 to 2006.