http://www.breitbart.com/big-government/2014/10/11/report-92-percent-of-mass-shootings-since-2009-occured-in-gun-free-zones/
The 'Crime Prevention Research Center' is neither a journal nor a peer-reviewed source of evidence. Considering that the website, the research, and the 'journal' are operated by the exact same guy, gun-rights advocate John Lott, there's a pretty enormous potential for experimental bias.
He's been called out by countless other academics for egregious experimental problems, but that kind of comes with the territory when you publish research that hasn't been peer-reviewed.
http://www.armedwithreason.com/shooting-down-the-gun-lobbys-favorite-academic-a-lott-of-lies/I'm fairly certain the reason we don't see civilian intervention in those kinds of incidents is because most mass shootings in the US happen in "gun-free zones", such as schools and universities, where shooters know there are practically no armed individuals around.
There's evidence that most mass-shootings target places that a shooter has a personal connection to, rather than a place that's gun-free. I'm willing to buy that gun-free zones probably don't dissuade shootings, but they aren't causing them either.
I looked online to see whether anyone has brown townyzed the outcomes of mass-shootings that have occurred in locations that allow guns, with armed citizens as bystanders. Turns out, it's pretty hard to find that kind of evidence because the internet is basically flooded with anecdote after anecdote about how "this mass shooting was stopped by this armed citizen" or "this mass shooting was almost made worse by this armed citizen".
Some website called Everytown Research did some similar research, but it suffers from basically the same experimental problems/lack of peer-review as CPRC (except biased
against guns, instead of
for them). Tomorrow morning, I'll see if I can find the type of study I'm looking for.