Measuring partisan bias in electoral systems with multi-member districts

GT 5.2 Sistemas electorales y representación

Pedro Riera Sagrera (Universidad Carlos III de Madrid)
Sesión 1
Día: jueves,21 de septiembre de 2017
Hora: 09:00 a 11:00
Lugar: Sala de Juntas

Partisan bias is said to exist when two parties with equal number of votes are allocated a different number of seats. It is thus a dyadic relationship not to be confounded with the traditional pro-majoritarian bias registered in all electoral systems in the world (i.e., the typical deviation from perfect proportionality). Partisan bias has been aptly studied for electoral systems with single-member districts, whereas for multi-member systems the literature has largely neglected this topic. We extend the idea of partisan bias to any kind of (single-tier) electoral system. Bias is measured as the additive result of an efficiency component (related to the heterogeneity of vote shares of main parties across districts), a vote-weight component (related mainly to malapportionment and differential turnout rates across districts) and third-party effects.

Palabras clave: electoral systems, partisan bias, comparative method