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We discuss conceptual issues arising from base rate neglect in the Efficiency Gap (Stephanopoulos and McGhee, 2015), a popular measure of unfairness in partisan gerrymandering. A measure of vote dilution asymmetry that adjusts for the base rate mitigates these; however, because it prioritizes a potentially unachievable degree of proportionality, its naive application may be impractical or infeasible. A comparison of measures using enacted and simulated plans clarifies the frequency with which the base rate issue may yield erroneous inferences if unaddressed. We show that a generalized version of dilution asymmetry that differentially weights harms associated with partisan gerrymandering permits and may even necessitate departures from pure proportionality. We also show via simulation that maps that approximate proportionality, and thus minimize dilution asymmetry, are feasible more often than previously thought.
Gordon et al. (Mon,) studied this question.