Fairness of seat allocation methods in proportional representation

Abstract

In this paper the fairness of some methods of allocating seats in a proportional representation (PR) voting system is investigated. Different PR systems are in use throughout the democratic world, but the primary focus here is the method used in South Africa, namely the largest remainder method with a Droop quota. It is shown that as the number of parties increases, the number of lost votes (votes not used to allocate seats) increases when using this method. Other existing allocation methods are discussed and compared with each other as well as with three optimisation methods (based on mathematical programming) introduced in this paper. Applying these mathematical programming methods results in allocations that are more fair than the existing methods of seat allocation, if South African voting data are used. These mathematical models attempt to minimise a number of different measures of the deviation between the actual percentage of votes received and the percentage of seats allocated to a certain party. Ideally this deviation should be zero, but due to the discrete nature of seats this is virtually impossible to achieve.

Pot and ladle: a formula for estimating the distribution of seats under the Jefferson–D’Hondt method

“…It is also employed, in combination with other methods, in Austria, Denmark, the Dominican Republic, Faroe Islands, Iceland, and Japan, and had been historically used in, among others, Bulgaria, France, Germany, Italy, Moldova, Norway, and Sweden. 3 It is well known that the Jefferson-D'Hondt method is biased in favor of larger parties (see e.g., Humphreys, 1911;Huntington, 1921;1928;1931;Morse, Von Neumann & Eisenhart, 1948;Rae, 1967;Taagepera & Laakso, 1980;Carstairs, 1980;Woodall, 1986;Taagepera & Shugart, 1989;Lijphart, 1990;Gallagher, 1991;Oyama & Ichimori 1995;Benoit, 2000;Balinski & Young, 2001: 72-74;Marshall, Olkin & Pukelsheim, 2002;van Eck et al, 2005;Pukelsheim, 2014). The magnitude of such bias has been estimated by Sainte-Laguë (1910), Pólya (1918aPólya ( , 1918bPólya ( , 1919aPólya ( , 1919bPólya ( , 1919c, Schuster et al (2003), Schwingenschlögl & Drton (2004), Drton & Schwingenschlögl (2005), Schwingenschlögl (2008), Pukelsheim (2014), and Janson (2014).…”

The Maltese Single Transferable Vote Experience: A Case Study of Gerrymandering?

“…Regarding the largest remainders, the performed methods are: Droop, Hangenbach-Bischoff, Hare, Imperial, and modified Imperial. For detailed information on the seat allocation methods, see (Herron et al, 2018, Van Eck et al, 2005.…”