The good news here is that the partisan diversity of a news site’s audience is an effective signal whether its audience is mostly Democrats or mostly Republicans. But it’s very effective on sites with mostly Republican audiences.

Check this chart: The blue line represents sites whose average user is a Democrat, the red line those whose average user is a Republican. The farther right you go, the more ideologically diverse its audience it.

Especially when you’re looking at it on a pageview level, there’s not a lot of movement in that blue line. The news sites Democrats read have a pretty consistent level of journalistic quality, whether they’re read only by Democrats or, say, 55% Democrats, 45% Republicans.

But the red line is dramatic. Sites that are read almost entirely by Republicans tend to be low in journalistic quality. But sites that are read mostly by Republicans — but also by a significant number of Democrats — tend to be very high in quality. (Call it the Wall Street Journal Effect.)