From the Violence Project website, on methodology:
Some sources define a mass shooting as any incident in which three or more victims were shot and killed, not including the perpetrator, while others use a threshold of four or more killed. Other sources define a mass shooting as any incident in which four or more people were shot or injured, which greatly increases the incident count. Some sources include domestic violence, gang conflict, drug trade disputes, or robberies in their numbers. At The Violence Project, we focus on mass public shootings, defined by the Congressional Research Service as follows:
“a multiple homicide incident in which four or more victims are murdered with firearms—not including the offender(s)—within one event, and at least some of the murders occurred in a public location or locations in close geographical proximity (e.g., a workplace, school, restaurant, or other public settings), and the murders are not attributable to any other underlying criminal activity or commonplace circumstance (armed robbery, criminal competition, insurance fraud, argument, or romantic triangle).”