for what it's worth, I suspect this definition of "mass shooter" is a lot closer to most people's understanding of the term than whatever your alternative stats are (which I'm guessing is any shooting where 2 or 3 people or more are shot).
I think most people imagine some maniac indiscriminately shooting a crowd of people he's never met before as opposed to a drug deal gone wrong etc where the victims may well be known to the assailant.