This rating is based on a measure of “ideological alignment” from an academic study - Bakshy, Messing, and Adamic (2015). This measure averages the self-reported ideology (where -2 is very liberal and 2 is very conservative) of users who shared an article from this source on a major social media website. “Alignment” is not a measure of media slant; rather, it captures differences in the kind of content shared among a set of partisans, which can include topic matter, framing, and slant.

Bakshy E., Messing S., Adamic L. (2015). Exposure to ideologically diverse news and opinion on Facebook. Science, 348, 1130–1132. doi:10.1126/science.aaa1160