Most of our federal and state legislators are elected from districts. Every ten years, state governments redraw district boundaries in a process known as redistricting. PlanScore promotes fairness in the redistricting process. We make it easy for policymakers and advocates to score new district maps and assess whether they’re fair or gerrymandered. We also provide access to the most comprehensive historical dataset of partisan gerrymandering ever assembled. As the 2021 redistricting cycle unfolds, PlanScore will collect and analyze new district plans as they are proposed in your state.
Gerrymandering is the process by which politicians draw voting maps to make it easier to get reelected. Through gerrymandering, a party is able to translate its votes into seats more efficiently than its opponent. Seats translate to legislative power and more control over the policies the government enacts.
Gerrymandering has been a practice in American elections nearly since the founding of the country. But in recent redistricting cycles, the accuracy of voter databases and mapping software has skyrocketed, leading to a trend of increasingly skewed maps in the states where politicians control district line drawing. One way to combat gerrymandering is through the creation of independent redistricting commissions, bodies separate from state legislatures that are responsible for drawing the districts. You can learn more about independent redistricting commissions in Campaign Legal Center's DemocracyU toolkit, and read the story of Nancy Wang, executive director of Voters Not Politicians, the group that spurred the creation of the Michigan Independent Redistricting Commission in 2018.
But first, in order to understand the impact of gerrymandering on our voting maps, we need to be able to measure it.
We use four distinct measures of partisan gerrymandering: the efficiency gap, partisan bias, the mean-median difference, and declination. All of these metrics are reliable when a state is competitive, but only the efficiency gap and declination 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 inefficient 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 inefficient. In the case of packing, all votes cast for the winning candidate, above the 50% (plus one) threshold needed for victory, are inefficient. The efficiency gap is calculated by taking one party’s total inefficient votes in an election, subtracting the other party’s total inefficient 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.
A gerrymander designs districts with the win/loss threshold in mind, to ensure that more districts of the favored party end up on the winning side of the threshold. A neutral map designs districts for other reasons, and does not treat the win/loss threshold as special. The declination metric treats threshold-related asymmetry in the distribution of votes across districts as indicative of partisan gerrymandering.