PolitiFact evaluates the truthfulness of an Obama ad

PolitiFact, a Tampa based organization  that evaluate the truthfulness of political claims or ads does a very interesting evaluation of an Obama ad about  how much Romney pays in taxes.  The ad says that Romney ‘s tax rate of 14% is probably less than you pay. 
PolitiFact says that if you include payroll taxes then the statement is true but if you only look at income taxes most low income people pay a lower rate.
Note that in the body of their article they estimate that even including payroll taxes Romney’s tax rate would still be 14%.
We should note one obstacle: We don’t know exactly how much Romney paid in payroll taxes. However, based on the amount and types of income listed on his tax return, even a generous estimate wouldn’t budge his overall tax rate much above 14 percent, so we think it’s reasonable to make this kind of calculation
But their conclusion about the truthfulness of the claim they say that Romney’s tax rate of 14% is based only on income taxes.   
There are two main ways to make this calculation, and they lead to opposite conclusions. While we believe that including payroll taxes in the calculation offers a more accurate picture of what the American public pays the IRS, it’s also true that the Obama ad didn’t specify which measurement it was using, and in fact used a figure for Romney — 14 percent — that was based on income taxes alone. On balance, then, we rate the claim Half True.
Apparently, they do not even read their own analysis in reaching their conclusions.