The Department of labor contracted a study in 2009 with Consad Research of Pittsburgh to study this very question. That study concluded that all but 5 to 7 cents (per dollar) of the wage gap could be explained by factors other than outright discrimination. The study did not conclude that 5 to 7 cents was caused by discrimination. It could have been caused by discrimination or other hard to identify factors. The important thing is that this study, was commissioned during Obama's Presidency. He knows that at most, gender discrimination in pay is 7 cents per dollar. He was happy to allow the impression that it is 28 cents, when he almost certainly knows otherwise.