PFB Smackdown: Ron Paul proves that PolitiFact isn't biased?

Ken Winters (on FaceBook) gave voice to one common type of defense of PolitiFact's objectivity:

Of the politicians checked by Politifact for more than 10 statements, the most honest one is Ron Paul and the most dishonest one is Michele Bachman (both are GOP). So anyone calling Politifact biased, needs to fact-check their own thinking.

I'll stick with the reply I offered on FaceBook:

You need to consider PF ratings in terms of selection bias (PF does not randomly choose its topics). It is completely consistent with a liberal bias to rate an oddball Republican (Paul) highly against mainstream Republican competition. Anyone who thinks that Paul's average rating on the "Truth-O-Meter" somehow gets PolitiFact off the hook from the charge of bias needs to fact check their own thinking.

Evaluations such as Winters' rely on dubious assumptions like the following:

• selection bias as to subject doesn't matter
• selection bias as to charitable/uncharitable interpretation doesn't exist
• selection bias as to charitable interpretation doesn't matter
• PolitiFact doesn't make a significant number of mistakes in its ratings

Winters went on to make a valid point in a subsequent post:  It's hard to back up the charge of bias empirically.

But it's just as hard to back up the claim of fairness empirically.