John Sides at the Monkey Cage discusses an article on public broadcasting and political knowledge. The cross-sectional survey data analyzed in the article cannot resolve the question of causal direction, as Sides notes:
Obviously, there are challenges of sorting out correlation and causation here. Do people who consume public broadcasting become more knowledgeable? Or are knowledgeable people just more likely to consume public broadcasting? Via statistical modeling, Soroka and colleagues go some distance in isolating the possible effects of public broadcasting—though they are clear that their modeling is no panacea.
Nevertheless, the results are interesting. In most countries, people who consume more public broadcasting know more about current events than people who consume less of it. But these same differences emerge to a lesser extent among those who consume more or less commercial broadcasting. This suggests that public broadcasting helps citizens learn. Here's a graph:
But the article should not be interpreted as providing evidence that "public broadcasting helps citizens learn."
Cross-sectional survey data cannot resolve the question of causal direction, but theory can: if we observe a correlation between, say, race and voting for a particular political party, we can rule out the possibility that voting for a particular political party is causing race.
Notice that in the United Kingdom, consumption of commercial broadcasting news correlates with a substantial decrease in political knowledge: therefore, if the figure is interpreted as evidence that broadcasting causes knowledge, then it is necessary to interpret the UK results as commercial broadcasting news in the UK causing people to have less political knowledge. I think that we can safely rule out that possibility.
The results presented in the figure are more likely to reflect self-selection: persons in the UK with more political knowledge choose to watch public broadcasting news, and persons in the UK with less political knowledge choose to watch commercial broadcasting news; that doesn't mean that public broadcasting has zero effect on political knowledge, but it does mean that the evidence presented in the figure does not provide enough information to assess the magnitude of the effect.