The official Bureau of Labor Department statistics show that the median earnings of full-time female workers is 77 percent of the median earnings of full-time male workers. But that is very different than “77 cents on the dollar for doing the same work as men.” The latter gives the impression that a man and a woman standing next to each other doing the same job for the same number of hours get paid different salaries. That’s not at all the case. “Full time” officially means 35 hours, but men work more hours than women. That’s the first problem: We could be comparing men working 40 hours to women working 35. …

But we’re still not close to measuring women “doing the same work as men.” For that, we’d have to adjust for many other factors that go into determining salary. Economists Francine Blau and Lawrence Kahn did that in a recent paper, “The Gender Pay Gap.”.”They first accounted for education and experience. That didn’t shift the gap very much, because women generally have at least as much and usually more education than men, and since the 1980s they have been gaining the experience. The fact that men are more likely to be in unions and have their salaries protected accounts for about 4 percent of the gap. The big differences are in occupation and industry. Women congregate in different professions than men do, and the largely male professions tend to be higher-paying. If you account for those differences, and then compare a woman and a man doing the same job, the pay gap narrows to 91 percent. So, you could accurately say in that Obama ad that, “women get paid 91 cents on the dollar for doing the same work as men.”