That issue was brought up indirectly when Jeff retweeted economist Howard Wall. Wall had tweeted:
.@jamestaranto I was an expert in Politifact once. Sole purpose of the interview was to illicit a few words to show pre-existing conclusion.
— Howard Wall (@GatewayEcon) January 10, 2016
We looked up the story where Wall was quoted as an expert. It was a fact check of Mitt Romney from the 2012 presidential election. The Romney campaign said women had suffered the greatest job losses under Obama, implying Obama's leadership had been bad for women.
PolitiFact ruled the claim "Mostly False."
The Romney campaign pushed back. PolitiFact looked at the issue again and ruled the claim "Mostly False." But at the same time, PolitiFact said "The numbers are accurate but quite misleading."
Don't blame the Romney campaign. It probably operated under the assumption that PolitiFact's definitions for its "Truth-O-Meter" ratings mean something.
Taking PolitiFact's definitions literally, the lowest rating one should receive for a literally true claim is "Mostly True." Once below that level, the definitions start talking about "partially true" statements that give a misleading impression ("Half True") and "The statement contains some element of truth" but ignores facts that could give a different impression ("Mostly False").
What's our point? We've always said PolitiFact's ratings reveal more about PolitiFact than they do about the entities receiving the ratings. It's a scandal that social scientists keep their eyes closed to that. Search PolitiFact's ratings for claims it says are literally true. Note the rating given to the claim. Then take a look at the ideology of the entity making the claim.
There's your evidence of journalistic bias by fact checkers.
This is an important issue. If social scientists aren't looking at it, it suggests they don't care.
Why wouldn't they care?
Jeff Adds: We highlighted a Mark Hemingway critique of PolitiFact's Romney claim back in 2012 that is still worth a read. It would seem little has changed at PolitiFact since then.
Update 0956PST 1/12/2016: Added "Jeff Adds" portion - Jeff