Partisan Bias

Partisan bias is the difference between each party’s seat share and 50% in a hypothetical, perfectly tied election. For example, if a party would win 55% of a plan’s districts if it received 50% of the statewide vote, then the plan would have a bias of 5% in this party’s favor. To calculate partisan bias, the observed vote share in each district is shifted by the amount necessary to simulate a tied statewide election. Each party’s seat share in this hypothetical election is then determined. The difference between each party’s seat share and 50% is partisan bias.

For more about partisan bias, see Bernard Grofman and Gary King’s 2007 Election Law Journal article, The Future of Partisan Symmetry as a Judicial Test for Partisan Gerrymandering After LULAC v. Perry.

Compare partisan bias to two other metrics, the efficiency gap and the mean-median difference.

Maryland 2012

Under Maryland’s 2012-2016 congressional plan, Democrats would win 25% extra seats in a hypothetical, perfectly tied election.

MD
+% D
Balanced
+% R

Alabama 2012

Under Alabama’s 2012-2016 congressional plan, Republicans would win 27% extra seats in a hypothetical, perfectly tied election.

AL
+% D
Balanced
+% R