A truth gap in gender pay?

From the White House to Pennsylvania's Democratic primary race, a gap in pay for men and women is once again getting of attention.

But it's also getting challenged.

President Obama this week again calls for action on a Paycheck Fairness Act requiring employers to disclose salary info, protect workers who inquire about same and make employers liable to pay discrimination civil suits.

The president again offers the stat that women make 77-cents for every dollar a man makes doing the same work.

And in Pennsylvania, Democratic frontrunner Tom Wolf is airing a campaign TV ad using the same stat, saying "We've got to do better. There's really no excuse."

There is, however, an argument over the stat.

A Washington Post fact check piece says the 77-cents number is based on Census Bureau annual wage data that does not consider hourly wages or salaried workers not paid on an hourly basis.

Annual wage data, for example, does not factor teachers who work nine months of the year and include, as Mitt Romney would say, binders full of women.

(You can read the full Post's "Fact Checker" here: It awards the President two "Pinocchios" for continuing to use the stat.)

The Post also notes more women work part time, leave the workforce for periods of time to have and raise children, seek jobs with flexible hours but lower pay and often enter careers with less compensation.

It notes a 2013 Georgetown University survey showing the highest valued college majors in terms of eventually making money -- engineering and science -- are dominated by males, while the 10 least valued for future bucks -- such as early childhood education and social work -- are dominated by females.

The point here is not that women don't work hard, almost always carry more burden in families or deserve equal pay for equal work.

The point is there are many factors that contribute to gender pay gaps, and using one statistic to argue a case against them isn't telling the whole story.

Presenting an argument based on selective data is, in this case, clearly good politics for Democrats. But it's clearly not the best way to build good policy.