None of them are CURRENTLY trying to be president, and if you look again, you'll see I removed Kerry from your quote. ;-)
I referenced exactly how they came to the UE numbers they posted, in the other thread.
US jobless rate falls to 7.8 pct., 44-month low
This says they ran a poll of Americans pretty much asking them if they had a job or not, with a pretty significant sample size. As long as they are consistent with their data collection method and data comparisons (I.e. they used the same method throughout the term) then I see no problem in using that figure as a reference.