PlanScore presents the most comprehensive historical dataset of partisan gerrymandering ever assembled. We also provide tools for policymakers and litigators to transparently score new plans and assess their fairness.
Partisan gerrymandering means drawing district lines to benefit one party and disadvantage the opposing party. Through gerrymandering, a party is able to translate its votes into seats more efficiently than its opponent. With more seats comes more legislative power and, ultimately, more control over the policies the government enacts.
We use three distinct measures of partisan gerrymandering: the efficiency gap, partisan bias, and the mean-median difference. All of these metrics are reliable when a state is competitive, but only the efficiency gap should be trusted when one party predominates in a state.
Partisan gerrymandering is always carried out by cracking a party’s supporters among many districts, in which their preferred candidates lose by relatively narrow margins; and/or by packing a party’s backers in a few districts, in which their preferred candidates win by enormous margins. Both cracking and packing produce votes that are wasted in the sense that they do not contribute to a candidate’s election. In the case of cracking, all votes cast for the losing candidate are wasted. In the case of packing, all votes cast for the winning candidate, above the 50% (plus one) threshold needed for victory, are wasted. The efficiency gap is calculated by taking one party’s total wasted votes in an election, subtracting the other party’s total wasted votes, and dividing by the total number of votes cast. It captures in a single number the extent to which district lines crack and pack one party’s voters more than the other party’s voters.
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.
The mean-median difference is a party’s median vote share minus its mean vote share, across all of a plan’s districts. For example, if a party has a median vote share of 45% and a mean vote share of 50%, then the plan has a mean-median difference of 5% against this party. When the mean and the median diverge significantly, the district distribution is skewed in favor of one party and against its opponent. Conversely, when the mean and the median are close, the district distribution is more symmetric.