“Mass Shootings: Definitions and Trends”, 2018-03-02 ():
There is no standard definition of what constitutes a mass shooting. Media outlets, academic researchers, and law enforcement agencies frequently use different definitions when discussing mass shootings, leading to different assessments of the frequency with which mass shootings occur and about whether mass shootings are more common now than they were a decade or two ago.
…These definitions matter. Depending on which data source is referenced, there were 7, 65, 332, or 371 mass shootings in the United States in 2015 (see table below), and those are just some examples. More-restrictive definitions (eg. Mother Jones) focus on the prevalence of higher-profile events motivated by mass murder, but they omit more-common incidents occurring in connection with domestic violence or criminal activity, which make up about 80% of mass shooting incidents with four or more fatally injured victims (2015).
…In 2014, the RAND released a study showing that “active shooting incidents” had increased at an average annual rate of 16% 2000–13201311ya (2014). In contrast to the varied definitions for mass shootings, there is an agreed-upon definition among government agencies for active shooter: “an individual actively engaged in killing or attempting to kill people in a confined and populated area; in most cases, active shooters use firearm(s) and there is no pattern or method to their selection of victims” (U.S. Department of Homeland Security 2008, pg2). Using a modified version of this definition to include incidents that had multiple offenders or occurred in confined spaces, 2014 found that active shootings had increased from only one incident in 2000 to 17 in 2013.
…In their analysis of mass shooting trends 1999–14201311ya, 2015 distinguished between mass shootings occurring in public locations that are indiscriminate in nature (“mass public shootings”), mass shootings in which the majority of victims are members of the offender’s family and that are not attributable to other criminal activity (“familicide mass shootings”), and mass shootings that occur in connection to some other criminal activity (“other felony mass shootings”). The two figures below show trends in these types of mass shooting incidents and fatalities, respectively, using the data provided in . Extending the data back to the 1970s, two studies found evidence of a slight increase in the frequency of mass public shootings over the past three decades ( et al 2014; 2015). However, using an expanded definition that includes domestic-related or felony-related killings, there is little evidence to suggest that mass shooting incidents or fatalities have increased (; ; 2016). Thus, different choices about how to define a mass shooting result in different findings for both the prevalence of these events at a given time and whether their frequency has changed over time.
…Definitional issues aside, the relative rarity of mass shooting events makes analysis of trends particularly difficult. Chance variability in the annual number of mass shooting incidents makes it challenging to discern a clear trend, and trend estimates will be sensitive to outliers and to the time frame chosen for analysis. For example, while 2015 found evidence of an upward trend in mass public shootings 1999–14201311ya, they noted that the increase was driven largely by 2012, which had an unusually high number of mass public shooting incidents. Additionally, 2015 showed that the RAND study’s estimate of a dramatic increase in active-shooter incidents was largely driven by the choice of 2000 as the starting date, because that year had an unusually low number of shooting incidents; extending the analysis to cover 1977 onward and adjusting the data to exclude events with fewer than two fatalities, found a much smaller and non-statistically-significant increase (less than 1% annually) in mass shooting fatalities over time.