30th mass shooting in the US already in 2018, we're only 45 days in.
How do we define mass shootings?
There is no widely accepted definition of mass shootings. People use either broad or restrictive definitions of mass shootings to reinforce their stance on gun control.
Researchers at the nonpartisan Congressional Research Service defined "mass shooting" as "a multiple homicide incident in which four or more victims are murdered with firearms, within one event, and in one or more locations in close proximity" in a 2015 report. They counted 317 mass shootings from 1999-2013.
The report also used the term "mass public shooting" for a multiple homicide incident in which four or more victims are murdered with firearms, within one event, in at least one or more public locations, such as a workplace, school or house of worship. There were 66 attacks that met that definition during that 15-year period.
After the 2012 Sandy Hook school shooting, Congress defined "mass killings" as three or more homicides in a single incident. The definition was intended to clarify when the U.S. Attorney General could assist state and local authorities in investigations of violent acts and shootings in places of public use.
The Gun Violence Archive tracks gun deaths using media, law enforcement, government and commercial sources. Its data is based on a broader criteria: at least four people injured or killed in one location, not including the suspect. Criminologists previously told PolitiFact that this group’s tally includes gang shootings and home invasion robberies.