How proportional are mixed compensatory electoral systems? Determining the necessary share of compensation mandates in mixed systems

Mixed compensatory systems (some call them “mixed-member proportional systems”), are praised for combining "the best of both worlds.” Yet, we know fairly little about their outcomes in practice. We still lack a genuine model that explains how many compensation seats are needed for the outcome to be proportional. Without this knowledge we will neither be able to estimate how many parties we expect to compete and to win representation in parliament under mixed compensatory systems; a key question of comparative research on electoral systems and party systems. This paper provides a quantitative model that shows under which conditions a mixed compensatory system might lead to proportional vote-seat conversion.