Tim Groseclose, a Professor at UCLA and Stanford and Jeff Milyo at the University of Chicago set out to statistical show whether or not major media outlets exhibited a conservative or liberal bias.

Their method was to count the number of times members of Congress cited various think tanks as "as if it were a disinterested expert on the topic at hand" and then compare it to the number of times major media cited these think tanks.

To compute our measure, we count the times that a media outlet cites various think tanks. We compare this with the times that members of Congress cite the same think tanks in their speeches on the floor of the House and Senate. By comparing the citation patterns we can construct an ADA score (a standard measure of political liberalism) for each media outlet.

As a simplified example, imagine that there were only two think tanks, one liberal and one conservative. Suppose that the New York Times cited the liberal think tank twice as often as the conservative one. Our method asks: What is the estimated ADA score of a member of Congress who exhibits the same frequency (2:1) in his or her speeches? This is the score that our method would assign to the New York Times.

So what were the results?

Although we expected to find that most media lean left, we were astounded by the degree. A norm among journalists is to present both sides of the issue. Consequently, while we expected members of Congress to cite primarily think tanks that are on the same side of the ideological spectrum as they are, we expected journalists to practice a much more balanced citation practice, even if the journalist's own ideology opposed the think tanks that he or she is sometimes citing. This was not always the case. Most of the mainstream media outlets that we examined (ie all those besides Drudge Report and Fox News Special Report) were closer to the average Democrat in Congress than they were to the median member of the House.

That's pretty damning stuff. You can see all the tables in the report, but I think it's most interesting to compare the estimated ADAs of various news outlets with specific members of Congress we all know (and er, love). Note, the ADA is based on 2002 voting and the higher the score, the more liberal.