A little while ago, Stu posted news on a study which demonstrates that the media are, in fact, biased. I (and several others here) read through it and thought that there were some flaws in the methodology. (Despite claims to the contrary, this is a valid form of critique!)
One of the central problems for scholars seeking to study ideological bias in the media is the lack of agreed-upon data. Natural scientists and even most social scientists can run experiments where their variables are to a considerable extent controlled. But this is simply impossible in the case of coverage of politics.
Now, none of this is to say that Alterman’s critique shows that the media aren’t liberal. (Though Alterman would argue this.) It is instead to simply show some of the major metholodigical flaws in the study, and to suggest that trying to get a fix on the politics of the media (made up of various forms (TV, newspapers, etc.), hundreds of outlets and tens of thousands of people from all walks of life, etc.) is very very difficult.
And please– actually read the article before posting on its contents.