Over the past week, I’ve repeatedly blogged about PlanScore’s upload feature, which allows users instantly to receive analyses of the partisan implications of proposed district maps. I also wanted to mention the site’s other half: comprehensive data about current and former plans’ partisan asymmetries. PlanScore currently covers congressional and state house plans from 1972 to the present (and it will be adding state senate plans soon). For each of these plans, the site reports three different measures of partisan asymmetry: the efficiency gap, partisan bias, and the mean-median difference. The site also indicates, for each metric, how the plan compares to the entire historical distribution.
The current data should be of interest to anyone who wants to know which maps around the country are (and aren’t) highly skewed. Litigators may note, for example, that the Ohio and South Carolina congressional plans, and the Florida and North Carolina state house plans, are about as asymmetric as the maps that have already been challenged on partisan gerrymandering grounds. It’s also notable that commission-drawn plans, as in California, New Jersey, and Washington, are substantially less skewed than maps designed by politicians.
The historical data is more for redistricting nerds than for the general public, but it still sheds light on many important cases and controversies. For instance: Why were Republicans upset enough about California’s 1982 congressional plan that they launched a (successful) statewide referendum to get rid of it? Because the plan was one of the most pro-Democratic on record, with a double-digit pro-Democratic efficiency gap. What happened after a court replaced Georgia’s 2002 state house plan—the subject of two Supreme Court cases—with a map of its own creation? The old plan’s large Democratic skew gave way to significantly greater partisan balance in the new map. And why were Texas Republicans so keen to re-redistrict the state’s congressional plan in 2004? Because the plan sharply benefited Democrats, and by redrawing it Republicans were able to shift its efficiency gap more than ten points in their favor.
This only scratches the surface of the data that is now available through PlanScore. To learn more, please poke around the site—and please let us know what additional analyses and capabilities you’d like to see us offer.