A study of mass shootings from 1950s to July 2016,  using FBI guidelines as to what qualifies as a mass shooting, was conducted by the Crime Prevention Research Center.
Amazingly, it showed that 98.4% occurred in places where citizens are not allowed to carry weapons, or "gun free zones."  Only 1.6% occurred in places where citizens were allowed to carry firearms.

Who did they use as the control group?

Who did they use as the control group?

There's no control group needed.  This isn't an experiment nor is anyone receiving medication or being affected in any way during the study.  A control group needs to be excluded from whatever factor is being tested.  If you disagree, then what would be the factor(s) you remove from the control group?

Like all of these studies, the general truth to keep in mind is, these stats do not demonstrate any causal effect.  Gun free zones don't cause mass murders, nor does having legal carry prevent it.  It does, however, illustrate a trend -- more mass shooters happen in gun free zones. 

There would need to be more data to determine if that was an intentional choice by the shooter.  Since many of them don't survive the shooting,  we'd need witnesses or suicide notes explaining whether they considered the risk of armed resistance at all.

The logical assumption is one would select a soft target area with little chance of self defense if the objective is maximum casualties.

That, again, is just an assumption.

The logical assumption is one would select a soft target area with little chance of self defense if the objective is maximum casualties.

That, again, is just an assumption.

An assumption that has the weight of evidence as substantiating: the probability of the dispersion of 98.4% to 1.6% being "random" is extremely low. Also, note that no "mass public shooter" so far has entered a police station, FBI or Secret Service office to conduct mayhem. No doubt some (all?) of these killers are mentally ill, but they aren't stupid.

An assumption that has the weight of evidence as substantiating: the probability of the dispersion of 98.4% to 1.6% being "random" is extremely low. Also, note that no "mass public shooter" so far has entered a police station, FBI or Secret Service office to conduct mayhem. No doubt some (all?) of these killers are mentally ill, but they aren't stupid.

If Everytown hadn't already trademarked the term with repetition, I would have said "Common  sense tells us it's more likely than not mass shooters choose victims with little or no means of resistance,"

but we all know who will be along to ask for links to my unbiased sources ....

If Everytown hadn't already trademarked the term with repetition, I would have said "Common  sense tells us it's more likely than not mass shooters choose victims with little or no means of resistance,"

There's also a "reason" that of all the crimes committed, very few are reported in the media, with the exception of every time a criminal tries to rob a gun store it makes the news. Whereas the same criminal with the same weapon stealing a purse from an 85 year old woman isn't media worthy. Self-explanatory and self-evident as an anecdotal case of "the exception proves the rule", but still not "proof" of causality.

Hence why FL is proposing a bill to end airport gun free zone.

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Hence why FL is proposing a bill to end airport gun free zone.

A radical notion: guns in airports! It'll be the Wild West! Well, except for the fact that 44 other states already allow guns in non-secure areas of airports, so Florida would be number 45. Obviously we don't have to guess which side of the fence Hawaii is on on this one... again.