One difficulty of gathering data on mass shootings is a lack of agreement on what qualifies as a mass shooting.
One common type of multiple murders is the familicide, or murder of an entire family, usually by an estranged family member. Another common type of multiple shooting involves gang-related or drug-related violence. Some sources count these incidents; others do not. Different sources also define "mass" or "multiple" with varying numbers of wounded or dead, usually at least 3 or 4. (For a discussion, see the "Methodology" section of "Mass Murder with Firearms," cited elsewhere on this page)
The type of mass shooting that attracts the most media attention is typified by the Sandy Hook Elementary School or Aurora, Colorado movie theater incidents, in which a single shooter kills or wounds multiple people in a public setting, apparently at random or without provocation. Attempts to define this sort of incident can be seen in the "active shooter" reports by the FBI and NYPD cited on this page.
There is a remarkable lack of consistent empirical data on mass shootings. Common government-compiled statistical sources on crime often lack the level of granularity or detail researchers would prefer in studying such a complex phenomenon. Other sources of data are compiled primarily from media reports and may suffer from inaccuracy and incompleteness.
For a review of these issues, see:
Although not updated, these reports by various agencies and organizations remain an important source of statistics.
Whether the rate of mass shootings is increasing is a frequent subject of media attention. The problem of definition directly affects this terms of this debate. How can we know whether these incidents are increasing or not if we cannot agree on what they are?
Mother Jones, the liberal political magazine that maintains an often-cited database on mass shooting statistics, and criminology scholars Dr. Grant Duwe and Dr. James Alan Fox have engaged in a public argument on this subject. Mother Jones claims the rates are increasing, Duwe and Fox claim they are not, and both sides fault each other's data.
According to the FBI's definition, mass shootings are increasing. For details, please see p.8 of their report, cited above: