Estimation of Electoral Disproportionality and Thresholds via MCMC

For statistical as well as political reasons combined measures of electoral disproportionality and electoral thresholds are necessary in order to adequately summarize electoral institutions. With few exceptions, none of these quantities can be reliably inferred directly from the provisions of the electoral law, thus impairing “large scale” comparative studies. Through the use of sampling based Bayes methods we are able to simultaneously estimate thresholds and disproportionality from electoral returns. We apply the proposed procedure on 45 electoral systems in use over 216 election to the national parliament in the 15 countries of the European Union in the period 1945-1996. The resultant two-dimensional summary of electoral systems has several advantages over measures of disproportionality currently used in comparative politics.