Hardly a day - hardly a post - goes by here at the Swing State Project without a reference to the Cook Political Report's Partisan Vote Index, or PVI for short. In the wake of the 2008 elections, SSP's pres-by-CD project has spurred a lot of discussion about how the PVI is calculated and why it's calculated the way it is.
Quite a few people people had a hard time believing my explanation of the math behind the PVI. But you don't have to take my word for it - this is how the Almanac of American Politics explains things:
Cook Partisan Voting Index. Refers to the Partisan Voting Index (PVI) as used by Charlie Cook, Washington's foremost political handicapper. The PVI is designed to provide a quick overall assessment of generic partisan strength. For this volume, the PVI includes an average of the 2000 and 2004 presidential elections in the district as the partisan indicator. The PVI value is calculated by a comparison of the district average for the party nominee, compared to the 2004 national value for the party nominee. The calculations are based upon the two-party vote. The national values for 2004 are George W. Bush 51.2% and John Kerry 48.8%. The PVI value indicates a district with a partisan base above the national value for that party's 2004 presidential nominee. Thus a district with an R+15 is a district that voted 15 percentage points (as an average of its 2000 and 2004 presidential vote) higher for Bush than the national value of 51.2%. Similarly, a district with a D+15 is a district that voted 15 percentage points (as an average of its 2000 and 2004 presidential vote) higher for Kerry than the national value of 48.8%. An X +00 indicates an evenly balanced district. (Emphasis added.)
The boldface sentences confirm my understanding of how PVI works. But why should it be calculated this way? I agree with the majority sentiment that it seems to make more sense to compare 2000 district performance with 2000 nationwide performance, not 2004 nationwide performance. This isn't as big of a deal with the two Bush elections because they were both so close, but comparing Kerry's 2004 district numbers with Obama's nationwide numbers produces some pretty serious gaps. I'd be curious to know what sort of justifications or rationales anyone can come up with for the status quo.
In the meantime, some have suggested computing an "SVI" - a "Swing State Project Voting Index," comparing 2004 to 2004 and 2008 to 2008. In fact, CalifornianInTexas has already gone ahead and started calculating these numbers. For the most part, these will be more favorable to Dems, as the big Kerry minus Obama splits are removed from the equation.
So, I'm asking the community: Should we use the "SVI"? Should it be in addition to the PVI? Are there any pitfalls if we do so? Any reasons not to? Let's hear your thoughts!