The fact that they use the propaganda term “gun violence” in describing the study, shows that they were not rigorous.
This type of study fails because it automatically excludes most of the successful defensive uses of firearms.
If three people are not killed, the event is not included in the study. Here are over 20 such events:
http://gunwatch.blogspot.com/2015/05/mass-killings-stopped-by-armed-citizens.html