Historical Data

PlanScore presents estimates of the efficiency gap, partisan bias, and the mean-median difference for congressional and state legislative district plans dating back to 1972. Simon Jackman, Eric McGhee, and Nicholas Stephanopoulos compiled these estimates in their academic and litigation-related work.

Not all states and years are included in PlanScore’s historical dataset. Congressional plans with fewer than seven seats (for which partisan asymmetry scores are less reliable) are omitted. So are state legislative plans with very high rates of uncontestedness or with unusual features such as multimember districts or runoffs.

The election results used to calculate the various measures of partisan asymmetry came from several sources. At the congressional level, we relied on datasets assembled by Gary Jacobson and Gary King. At the state legislative level, Carl Klarner provided us with all of the relevant data.


Assessing the Current Wisconsin State Legislative Districting Plan, Simon Jackman’s 2015 expert witness report in Gill v. Whitford, analyzing state legislative plans.

Assessing the Current North Carolina Congressional Districting Plan, Simon Jackman’s 2017 expert witness report in LWVNC v. Rucho, analyzing congressional plans.