Polarization

Polarization is a measure of how likely the districts in a plan will stay with their parties. A low polarization score indicates that the districts in a plan are more responsive to changes in the electoral mood and reflects the overall competitiveness of the districts.

The polarization score is based on the PVI of the districts in a plan:

For each highly competitive district (PVI 0 or 1) add 0

For each competitive district (PVI 2 through 5) add +1

For each noncompetitive district (PVI 6 or greater) add +2

The total is the POLARIZATION score for the plan.

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For example consider the Congressional delegation in 2016. The PVIs are based on the 2008 and 2012 results for the districts as they exist in 2016: PA-1: D+28 PA-2: D+38 PA-3: R+8 PA-4: R+9 PA-5: R+8 PA-6: R+2 PA-7: R+2 PA-8: R+1 PA-9: R+14 PA-10: R+12 PA-11: R+6 PA-12: R+9 PA-13: D+13 PA-14: D+15 PA-15: R+2 PA-16: R+4 PA-17: D+4 PA-18: R+10

There are 4 noncompetitive districts for Democrats (PA-1,2,13,14) There is 1 competitive district for Democrats (PA-17) There is one highly competitive district (PA-8) There are 4 competitive districts for Republicans (PA-6,7,15,16) There are 8 noncompetitive districts for Republicans (PA-3,4,5,9,10,11,12,18)

This can be written in shorthand as (4D, 1d, 1e, 4r, 8R).

The score is 0*1 + (1+4)*1 + (4+8)*2 = 29. This is the POLARIZATION.