Does anyone have any links regarding the amount of mass shootings that happen in gun free zones?

I'm seeing a lot of different numbers regarding the mass shootings in gun free zones. I'll find one link saying over 90% of mass shootings happen in gun free zones but then another saying that less than 20% of mass shooting happen in gun free zones. Is there any mostly unbiased links regarding this issue? via /r/CCW http://ift.tt/2jvH8r3