Several different inclusion criteria are used; there is no generally accepted definition.
Gun Violence Archive, a nonprofit research group that tracks shootings and their characteristics in the United States, defines a mass shooting as an incident in which four or more people, excluding the perpetrator(s), are shot in one location at roughly the same time. A 2019 study of mass shootings published in the journal
Injury Epidemiology recommended developing "a standard definition that considers both fatalities and nonfatalities to most appropriately convey the burden of mass shootings on gun violence." The authors of the study further suggested that "the definition of mass shooting should be four or more people, excluding the shooter, who are shot in a single event regardless of the motive, setting or number of deaths." Definitions generally exclude consideration of the number of persons targeted with
lethal intent, perhaps with degraded accuracy from a greater distance, who escape injury from bullets or bullet spall, regardless of injury sustained while evading live gunfire, or medical complications resulting from those injuries (such as infection, concussion, stroke, or PTSD) further down the road. Unless a newspaper article or other source for the shooting states the shooting may have involved gang activity, a robbery, organized crime, etc., it is not considered a gang-related, robbery-related, or organized crime-related shooting to be excluded from the list under some of the definitions above. Shootings that are reported as occurring in a home, at a party, or in an apartment complex with no other information given are considered non-public shootings and are excluded by some of the definitions above that state the shooting occurred "in a public place." For statistical purposes, armed accomplices are likely to be classified as perpetrators, even if later analysis determines that the accomplice never discharged a firearm. Bystanders struck by bullets fired in self-defense by another bystander would potentially be classified as victims of a mass shooting, while a bystander firing in self-defense who injures or kills another bystander would almost certainly not be classified as a perpetrator. The classification of a bystander struck by police while attempting to take out a believed perpetrator falls into a gray zone. == List ==