Calculation
PVIs are calculated by comparing the district's average Democratic (or Republican) Party's share of the two-party presidential vote in the past two presidential elections to the nation's average share of the same. The national result was 51.2% Democratic, 48.8% Republican for the average of 2004 and 2008. For an example using rounded numbers, in Alaska's At-large congressional district, the Republican candidate won 63% and 61% of the two-party share in the 2004 and 2008 presidential elections, respectively. Comparing the average of these two results (62%) against the average national share (49%), we find this district is 13% more Republican than the country as a whole, or R+13.
Prior to its April 2009 update, the PVI formula was calculated by comparing district-level results for the past two presidential elections to nationwide results for only the most recent election. David Nir of the Swing State Project advocated a change to the new formula, and Charlie Cook agreed, wanting an "apples to apples" comparison.
Read more about this topic: Cook Partisan Voting Index
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