"A Measure of Media Bias" is a research study headed by Tim Groseclose (formerly of the Department of Political Science, UCLA; currently a professor of economics, George Mason University) and Jeff Milyo (Department of Economics, University of Missouri).
The study authors measured media bias by estimating ideological scores for several major media outlets. Groseclose developed a measure, the Slant Quotient (S.Q.), a number that shows how often a news outlet cites one or more of about 200 think tanks. The bigger the S.Q., the more liberal the news outlet.
Among his findings: the Wall Street Journal news section is left of center, with a 55.1 SQ. The most liberal newspaper is the Detroit Free Press (S.Q. = 81.5) and the New York Times is very liberal (67.3). AllSides takes Groseclose's findings into account when determining our media bias ratings, but this study alone does not determine our ratings — we have multiple methodologies for rating bias.