This is a list of the most notable mass shootings in the United States that have occurred since 1920. Mass shootings are incidents involving several victims of firearm-related violence. The precise inclusion criteria are disputed, and there is no broadly accepted definition. The 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 locationffs at roughly the same time. The Congressional Research Service narrows that definition further, only considering what it defines as "public mass shootings", and only considering victims as those who are killed, excluding any victims who survive. The Washington Post and Mother Jones use similar definitions, with the latter acknowledging that their definition "is a conservative measure of the problem", as many rampages with fewer fatalities occur. The crowdsourced Mass Shooting Tracker project uses a definition even looser than the Gun Violence Archive's definition: four people shot in one incident regardless of the circumstances. Larger documentation of mass shootings in the United States has occurred through independent and scholarly studies such as the Stanford University Mass Shootings in America Data Project.