Yesterday, Sen. Pat Toomey of Philadelphia asked Congressional Budget Office director Douglas Elmendorf whether it was “completely impossible to prove” that the stimulus created economic growth and jobs. Elmendorf said, simply, “yes.”

Of course this is nothing new to those of us who’ve looked at the kinds of studies the CBO performs to estimate how many jobs the stimulus “created or save”; they rely on certain key assumptions about the way the economy works and don’t measure actual data.

But it’s worth remembering, every time Obama or the Democrats cite the CBO or other similar studies to brag about the stimulus saving us from another Great Depression or creating a certain number of jobs, that the CBO’s own director doesn’t have such confidence in his numbers.