The study concludes that media sources have a significant impact on the number of questions that people were able to answer correctly. The largest effect is that of Fox News: all else being equal, someone who watched only Fox News would be expected to answer just 1.04 domestic questions correctly — a figure which is significantly worse than if they had reported watching no media at all. On the other hand, if they listened only to NPR, they would be expected to answer 1.51 questions correctly; viewers of Sunday morning talk shows fare similarly well. And people watching only The Daily Show with Jon Stewart could answer about 1.42 questions correctly.
“These differences may be small, but even small differences are important when we’re talking about millions of people”
The study claims that partisan media is to blame—they say MSNBC is almost as bad as Fox—but I think it has more to do with credulousness. NPR and The Daily Show are both much more likely to question the conventional wisdom they're reporting, and the Sunday morning talk shows at least spend a lot of time on a single subject, even if they are echo chambers.