The “righteous vengeance gun attack” is just one of 10 types of terrorism identified by Chenoweth and Lowham via statistical clustering techniques. Another cluster is “bombings of a public population where a liberation group takes responsibility.” You can read about the 10 clusters, and the 44 dichotomous variables (suicide or not, bombing or not, religious site or not, etc.) that were used in the cluster analysis here – http://www.ericachenoweth.com/wp-content/uploads/2012/01/ChenowethLowham.pdf. Learn more about the statistical underpinnings of cluster analysis and its implementation in Anthony Babinec’s online course Cluster Analysis,  at Statistics.com.