The partisan voting index or PVI is a standard measure of the political tendencies and competitiveness of a district. It is based on methodology originally developed for the Cook Political Report. The PVI measures the 2-party vote in a presidential race in a district compared to the national result. In principle it predicts how a district would vote in a election when the major parties had equal vote shares nationally.
The PVI is expressed as a percentage shift in favor of one party or another within a state or district. For example if a district would be expected to vote 5% more for the Democratic candidate than predicted by national results, the district would be labelled D+5. If the same shift were instead in favor of the Republicans the district would be R+5.
The PVI is only based on the votes cast for the two major parties in presidential election. The Democratic share of the 2-party vote is found by taking the presidential vote for the Democrat and dividing it by the total vote for the Democrat and Republican. This can be done with percentages of the vote as well as actual vote totals. Nationally the Democratic share was 0.488 (48.8%) in 2004, 0.537 (53.7%) in 2008, and 0.520 (52.0%) in 2012.
The PVI for a district for an election is found by taking the Democratic share for the district in that election and subtracting the national share. Officially the Cook PVI is equal to the average of the PVIs for the two previous elections expressed as a percent. PVIs are usually rounded to the nearest whole number. Since the above calculation was based on the Democratic vote share a negative PVI means that a district favors the Republican, so negative PVIs are written as positive PVIs for the Republicans. For example a PVI = -0.026 = -2.6% is expressed as R+3.