Explaining the PVI metric
Recently we’ve been asked questions about the Cook PVI, or Partisan Voting Index, used in DistrictBuilder. The Cook PVI is the competitiveness metric that we provide in the Evaluate panel. Essentially, it’s a measurement of how strongly a district leans towards the Democratic or Republican party, compared to the national average. It’s displayed with the Party letter (R or D) followed by a number reflecting the partisan preference in percentage points. For example, R+1 or D+4.
How is it calculated?
First and foremost, to calculate the Cook PVI you need Presidential election results from the two most recent elections. For most states, we have 2016 and 2020 data. We continue to expand the political data available thanks to our data partner, the Voting and Election Science Team (VEST). Since the PVI compares how the district voted to the national average, we use the national two-party vote from 2016 and 2020. The baseline is the national average, seen below (thanks to DailyKos for providing this handy spreadsheet):
Next, we aggregate the Presidential election results to the block-level Census data in DistrictBuilder. Let’s take the proposed TX-23rd Congressional district along the Texas panhandle. The district has a PVI of R+5. That means it’s 5 percentage points more Republican than the national average. Here’s what that looks like in DistrictBuilder:
The Cook PVI uses the two-party vote. When you hover over the map, the tooltip will show you the major party and include the other party as part of the percentages. The tooltip over the metric viewer will only show you the two-party vote but still include the other party vote. The two-party vote is:
The average of the two elections is 53% Republican, which is 5% more Republican than the national average of 48% Republican, hence R+5.
Why pay attention to the Cook PVI?
In general, it can be considered good practice to ensure that your district plan has a similar partisan lean to the state as a whole. While partisan gerrymandering isn’t banned at the federal level, many states consider partisan balance an important criterion in redistricting.
