This research note engages the current research on measuring media bias. I present a reanalysis of the results found in Groseclose and Milyo (2005) and show that the original parameter estimates of the ideological positions of media outlets are not stable over time. Using the same data but analyzed over different periods of time, I find a different conclusion than the previous article. I examine four-year rolling time periods and find that the data produce different parameter estimates in the early- to mid-1990s as compared to after 2000, with all analyzed outlets appearing more moderate or conservative in later time periods. My results indicate that the estimated positions are sensitive observations in the data and the time period of observation of the outlet.
The GM finding of a strong liberal bias seems suspect in light of my reanalysis. The findings presented here indicate liberal media outlets during the early 1990s, but conservative media outlets by the beginning of the next decade.
And the results also change if one interest group (the National Taxpayers Union) is excluded from the estimation of the positions of media outlets:
Performing a pooled analysis without using citations of the National Taxpayers Union (NTU), the most frequently cited policy group by members of Congress, resulted in an average drop in nearly nine points on the ADA scale. In other words, excluding citations of the NTU from both members of Congress and media outlets results in all media outlets appearing substantially more conservative.
For more, see the paper as well as Andy’s post on the Groseclose book (and the comments), the Puglisi-Snyder paper, Andy’s comments on that paper, Brendan Nyhan’s post on the original Groseclose-Milyo paper, and my earlier post on that paper and the general subject of ideological bias in the news media.