PlanScore bases its scores on predicted precinct-level votes for each office: 2018 State House and 2018 State Senate. We generate these predicted votes using demographic and political variables entered into an ordinary least squares regression model.
To predict turnout we regress total major-party vote for the race in question on total major-party 2016 presidential vote. To predict vote share we regress the Democratic share of the major-party 2018 vote on the Democratic share of the major-party 2016 presidential vote. Using the coefficients and standard errors from these models, we then generate 1,000 simulated total votes and Democratic vote shares for each precinct. These numbers are the inputs for calculating 1,000 sets of efficiency gaps, partisan biases, and mean-median differences, which produce the means and margins of error reported on PlanScore.org.
Complete model code is available in PlanScore/Model-Generator.
