Five variables were studied, and unionization was the one found to be least significant statistically. That's not what most observers and commentators in this community would have expected. Researchers found more correlation with salary levels.
To their credit, Beth Kellar's group at SLGE and their researchers have done some notable and informative work in the field. One does not need to agree with every finding to have immense respect for their research and efforts to illuminate the public policy debates.
As the report has just been released, I haven't had time to carefully study the whole thing, but I wanted readers to be aware of its availability. It's not light summer reading, but it's worthy of your time.