What is the Cook Partisan Voting Index or PVI?

The Cook Partisan Voting Index (PVI) evaluates partisan entrenchment in each state by considering the past two presidential elections as a proxy.

It looks at the nationwide preference for major party candidates in the two most recent presidential races and then looks at how many percentage points the state deviates from the national outcome. In the 2017 report, Minnesota preferred Democratic candidates by 1% (PVI was D+1), while Indiana preferred Republican candidates by 9% (PVI was R+9).