So what classifies a think tank or policy group as conservative/liberal/or centre?
Most call themselves non-partisan
.
As for this:
the average ADA score in Congress (50.1)
Ummm, Congress is right wing (you know, more Republicans). At this stage, the study incorrectly appears to assume that the average congress vote = the average US voter (ok) = "centre" (not ok).
Centre isn't a US concept (a bit like the World Series isn't the world). A truely centrist media would be centrist across its whole audience, not just the US.
But really, I need to see the whole article - there is no indication in that news release about the significance of the results.