I found a summary of this article on the NFOA forum. Nebraska residents... it's free to join and a wealth of knowledgeable people.
A non-firearms owner did a data analysis of 29 mass shootings specifically looking at shootings where police ended the rampage, or civilians (armed or unarmed) ended it. What he found was that in shootings where the victims waited for police to arrive to save the day, ended in an average of 14.3 dead (200 dead in 14 incidents). In the instances where an armed (5 times) or an unarmed (10 times) citizen took quick action, an average of 2.3 people were killed (35 dead in 15 incidents). You do the math.
As a husband and father, I'm interested in anything that helps educate me in the area of self-defense, and I'm always watching for products that help me be prepared.