(Cross-posted at Congress Matters)
The Cook Political Report's Partisan Voting Index (PVI) is a measurement of how strongly an American congressional district leans toward one political party compared to the nation as a whole. The index value for each congressional district is derived by averaging its results from the prior two presidential elections and comparing them to national results. The resulting PVI indicates which party's candidate was more successful in that district as well as the number of percentage points by which its results exceeded the national average. By using two elections this method smooths out bumps in election results that may occur because particular candidates played anomalously well or poorly in particular regions.
By trying to calculate the new PVIs after the 2008 elections it turns out that Cook's method has serious flaws and there is currently a discussion going on about this over at the Swing State Project. In my diary I want to showcase what seems to be the best approach to solve the problem.
