Introduced by the Cook Political Report in 1997, the Cook Partisan Voting Index (Cook PVI) measures the partisan lean of each congressional district by comparing its presidential vote share to the national presidential vote share, averaged over the two most recent elections. A district rated R+10 voted 10 percentage points more Republican than the national average. Districts rated between D+5 and R+5 are considered competitive swing seats. Cook PVI is the primary nonpartisan tool used by political analysts, campaigns, and media to assess district competitiveness and interpret special election results in context.