Cook Partisan Voting Index



The Cook Partisan Voting Index, abbreviated PVI or CPVI, is a measurement of how partisan a United States congressional district or state is. This partisanship is indicated as lean towards either the Republican Party or the Democratic Party, compared to the nation as a whole, based on how that district or state voted in the previous two presidential elections.

History
The Partisan Voting Index was developed in 1997 by Charlie Cook of The Cook Political Report, in conjunction with Clark Bensen and his political statistical analysis firm, Polidata, "as a means of providing a more accurate picture of the competitiveness". It is based on the methodology introduced by Rob Richie of the Center for Voting and Democracy (FairVote for the Center's July 1997 Monopoly Politics report.

The Cook Political Report has since released new PVI scores every two years. In 2021, the newsletter ended its relationship with Polidata and instead used Dave Leip's Atlas of U.S. Presidential Elections to calculate 2020's results. The most recent iteration is the 2022 Cook Partisan Voting Index, which was released with an updated formula for calculating PVI values.

Calculation and format
The index looks at how every congressional district voted in the past two presidential elections combined and compares it to the national average. The Cook PVI is displayed as a letter, a plus sign, and a number, with the letter (either a D for Democratic or an R for Republican) indicating the party that outperformed in the district and the number showing how many percentage points above the national average it received. In 2022, the formula was updated to weigh the most recent presidential election more heavily than the prior election.

By congressional district
The PVIs for congressional districts are calculated based on the 2016 and 2020 presidential elections. With a PVI of R+1, MI's 8th congressional district was determined to be the median congressional district, meaning that exactly 217 districts are more Democratic and 217 are more Republican than this district. , in the House, there are 222 districts more Republican than the national average, and 206 districts more Democratic than the national average. The number of swing seats, defined as those between D+5 and R+5, is 87.

By state
The PVIs for states are calculated based on the results of the U.S. presidential elections in 2016 and 2020. The table below reflects the current state of Congress and governors, based on the most recent election results.