Ezra Klein is too smart to be writing posts like this:

The new Pew Poll on public knowledge of current affairs includes the sadly routine finding that Fox News is doing a remarkable amount of nothing for its viewing audience. Subjected to 35 questions about the news, regular viewers of Fox scored directly in the national average, showing no sign of enhanced knowledge for all the time spent before Brit Hume. Blogs, too, appeared to do little good for their audience, lifting scores by only 2%. The Daily Show and Colbert Report either attract or educated the most informed viewers, along with newspaper websites, PBS, NPR, Limbaugh, and O’Reilly. Maybe none of this should be surprising, though. In the end, Fox News doesn’t exist to inform — it exists to convince. And in that, it’s doing just fine.

Actually, we can’t infer anything about whether Fox informs its viewers from these data. We have no way of knowing what level of knowledge Fox viewers would have had if they hadn’t watched Fox (all else equal).

To illustrate the point, imagine trying to measure the effect of Sesame Street on children’s reading or vocabulary. You can’t just compare the reading levels of kids who watched Sesame Street and those that don’t. The reason is that the socioeconomic status of the children (or their parents’ education level, etc.) is likely to be correlated with viewing of Sesame Street. The same principle applies here.

Klein implicitly acknowledges the possibility that audiences select the media they consume later in the post, writing that “The Daily Show and Colbert Report either attract or educated the most informed viewers” (my italics). I’ll bet on “attract.” While I’m no fan of Fox News, this is an ill-informed criticism at best.