from the above article: "There is no evidence to support the argument that arming good guys is a good idea. We studied scores of these shootings and in the last 30 years, not a single shooting has been stopped by an armed citizen. Conversely, there's evidence it is a bad idea."
these people claiming the above obviously don't have their heads on straight. of course it's difficult to collect evidence on this matter, because if a shooting is stopped by an armed citizen then there is no shooting to raise attention and be studied!!! the only massacres that have occurred are ones where armed citizens were not present to stop them!!!
for example: a shooter tried to mass murder in a mall, and was unsuccessful due to a 22 year old male citizen being armed. the shooter killed only 1 or 2 people, then when the citizen pulled a gun on the shooter, the shooter ended his own life, and the citizen never even had to pull the trigger. but guess what, this wasn't included in this study, because there was no massacre; BECAUSE IT WAS STOPPED BEFORE IT COULD BECOME A MASSACRE, BY AN ARMED CITIZEN!!