From the Executive Summary: "Deadly mass shootings are a pressing concern among Americans. Nevertheless, reliable information on the extent and nature of such incidents is often lacking, which may undermine the effectiveness of any 'evidence-based' policy efforts aimed at combating such events. Under NIJ's [National Institute of Justice's] long-standing program of research related to firearms violence and prevention, and the 'knowledge building' area within the program, the purpose of the study was to compile a database to better understand a broad array of deadly mass shootings in America over the past four decades. Specifically, our database was created to assess features and trends in all deadly mass shootings in America between 1980 and 2018. In these data, mass shootings encompass 'all incidents with four or more gunshot fatalities, not including the shooter, within 24 hours'. [...] Our effort to be more inclusive allows for the examination of not only the characteristics of deadly mass shootings that occur in public, but to also examine how similar (or not) these incidents are to deadly mass shootings that take place in other contexts. In all, our database contains information on 720 incidents that occurred in both public and private spaces between 1980 and 2018. Consistent with prior research, we used open-source data (e.g., media reports, official police and court records) to code each deadly mass shooting incident according to a host of characteristics."
National Criminal Justice Reference Service Document Number 305090
Office of Justice Programs: https://www.ojp.gov/