Oh, okay, old age is a bitch, I remembered something correctly : )
They did a meta-analysis of other studies
For example, one study showed that raising minimum wage increased unemployment, but they didn't control for regional differences in employment. The rise in unemplyment was actually due to differences in different regions that had nothing to do with minimum wage.
So then, like you say, people that want to argue that raising minimum wage will increase unemployment will trot out that study and the opposite side a different study...
Now, unless I get bored, I will look for a study that shows the effect on the economy.