Jim Manzi's post on the much-discussed Bartels correlation is extremely illuminating:
Bartels’s thesis is primarily the statistical artifact of the combination of two very simple observations: (A) there has been a higher proportion of Republican presidential years during the period 1980 – 2005 than the period 1948 – 1979 (64% vs. 48%), and (B) starting in 1979, US income inequality began to rise dramatically after a post-WW II period of relative wage equality. Accepting these two statements of fact does not imply accepting Bartels’s thesis that A caused B, and his academic paper on the subject provides no compelling evidence of causality.
It also turns out that Bartels' result depends heavily on his choice to give presidents credit for ONE year after their term. If you credit them for just the actual years of their term, or for two years after their term, the result all-but-vanishes. There is a lot more in Jim's post, all of it excellent, so do check it out.