The study compares how often--in news articles or reports--each outlet cites certain think tanks vs. how often members of Congress cite them. Using the ADA (Americans for Democratic Action) ratings for each member of Congress a theoretical ADA rating is computed for each outlet.
(The ADA, an ultra-liberal organization, takes 20 votes a year in both the House and Senate and gives a legislator 5 points if they vote the the way the ADA wants. For example, Ted Kennedy has about a 90 lifetime rating and Orrin Hatch, about 5)
What is excellent about this study is it is pretty objective - conservatives cite certain think tanks much more than liberals and vice versa. Read it here.
Excerpt (from the study):
Our results show a very significant liberal bias. All of the news outlets except Fox News Special Report received a score to the left of the average member of Congress. Moreover, by one of our measures all but three of these media outlets (Special Report, the Drudge Report, and ABC's World News Tonight) were closer to the average Democrat in Congress than to the median member of the House of Representatives. One of our measures found that the Drudge Report is the most centrist of all media outlets in our sample. Our other measure found that Fox News Special Report is the most centrist. These findings refer strictly to the news stories of the outlets. That is, we omitted editorials, book reviews, and letters to the editor from our sample.
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.