Hi Tim -
I haven't read your book yet, but I was made aware of it yesterday through David Henderson's review in Regulation magazine. So my understanding of what you were doing is based on that and a review of your QJE article on the slant quotient.
I had a question about your approach that I just couldn't figure out and I was hoping you could explain it to me. Given that it's the media's job to report facts impartially, and given what we know about how politicians present facts to win votes and funding, why are you judging the bias of the media using the standard of the ideological composition of politicians? That seems backwards to me, so I'm wondering if I've missed something. One would think - when observing a difference between media views and politician views - that it's probably indicative that politicians are biased. I would have thought the title of your book should be "Right Turn".
Now of course this approach isn't perfect either. It's likely that both are biased to a certain extent, and therefore it's impossible to pin down exactly who is biased in which direction. But even in that case, I'm not sure how you can confidently claim that the media has a liberal bias.
Perhaps I should put it this way: how would you arbitrate between the claim (1.) politicians have a conservative/libertarian bias and (2.) the media has a liberal bias. You might address this in the book (and I plan on picking it up and reading it once I'm out of classes this summer). But at the moment I'm not sure why you opt for (2.) instead of (1.).
If I had to arbitrate between the two, I'd do two things. First, I'd focus on cases where there was a clear "right" answer - or at least a strong consensus - so that my measure of bias is nailed down better.
Second, I'd talk a lot about incentives and public choice type issues - and make a theoretical argument for why it makes more sense to say "politicians have a conservative/libertarian bias" than it does to say "the media has a liberal bias". Both may be a little true (or maybe the truth is so far left that both the media and politicians have a conservative/libertarian bias). But that would help to get at it. It's like Bryan Caplan's point about voters. When there's a difference in opinion between the experts and the general public, you probably don't want to argue that the experts are biased because they don't conform to the general public. I would probably choose to avoid the assertions about rationality or irrationality about why the two populations diverge.