On the basis of a formula for calculating seat shares and natural thresholds in multidistrict elections under the Jefferson-D’Hondt system and a probabilistic model of electoral behavior based on Pólya’s urn model, we propose a new model of the relationship between the district magnitude and the number / effective number of relevant parties. We test that model on both electoral results from multiple countries employing the D’Hondt method (relatively small number of countries and elections, but wide diversity of political configurations) and data based on hundreds of Polish local elections (large number of countries and elections, but much higher degree of parameter uniformity). We also explore some applications of the proposed model, demonstrating how it can be used to estimate the potential effects of electoral engineering
