A generalized parametric divisor method for political apportionment

Abstract

AbstractThe political apportionment problem has been studied for more than 200 years. In this paper, we introduce a generalized parametric divisor method (GPDM), which generalizes most of the classical and widely used apportionment methods from the literature as special cases. Moreover, it allows for very flexible interpolation between previous methods by appropriately setting two parameters in the GPDM. We identify an inequity measure that the GPDM globally optimizes. We also identify two natural inequity measures for which an apportionment given by the GPDM is locally optimal. These results generalize similar results for classical apportionment methods, and justify the use of a large class of new apportionment methods given by the GPDM. From this class, we identify and recommend specific new methods. Our numerical experiments compare the apportionments given by the new methods with those given by existing methods using real data for the United States, Germany, Canada, Australia, England, and Japan. Explicit definition of the GPDM has enabled us to perform computational experiments for evaluating the unbiasedness of the GPDM using two standard measures while comparing with other traditional methods. Based upon our generalization technique and numerical experiments, we show that the GPDM outperforms all the traditional apportionment methods by selecting appropriate parameter values. Thus, we can conclude that the GPDM is the most “unbiased” and fairer if parameters can be agreed ex ante, and the GPDM is applicable to actual electoral voting systems.