Seat bias is a property describing methods of apportionment. These are methods used to allocate seats in a parliament among federal states or among political parties. A method is biased if it systematically favors small parties over large parties, or vice versa. There are several mathematical measures of bias, which can disagree slightly, but all measures broadly agree that rules based on Droop's quota or Jefferson's method are strongly biased in favor of large parties, while rules based on Webster's method, Hill's method, or Hare's quota have low levels of bias, with the differences being sufficiently small that different definitions of bias produce different results.
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👉 Seat bias in the context of Quota rule
In mathematics and political science, the quota rule describes a desired property of proportional apportionment methods. It says that the number of seats allocated to a party should be equal to their entitlement plus or minus one. The ideal number of seats for a party, called their seat entitlement, is calculated by multiplying each party's share of the vote by the total number of seats. Equivalently, it is equal to the number of votes divided by the Hare quota. For example, if a party receives 10.56% of the vote, and there are 100 seats in a parliament, the quota rule says that when all seats are allotted, the party may get either 10 or 11 seats. The most common apportionment methods (the highest averages methods) violate the quota rule in situations where upholding it would cause a population paradox, although unbiased apportionment rules like Webster's method do so only rarely.
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Seat bias in the context of Hare quota
The Hare quota (sometimes called the simple, ideal, or Hamilton quota) is the number of voters represented by each legislator in an idealized system of proportional representation where every vote is used to elect someone. The Hare quota is equal to the number of votes divided by the number of seats.
The Hare quota was used in Thomas Hare's proposal for a single transferable vote system and can still be used for this purpose, though the Droop quota is used for most STV elections today. The Hare quota is often used to set electoral thresholds and to calculate apportionments under party-list proportional representation when using the largest remainder method. In such cases, the Hare quota gives unbiased apportionments that do not favor either large or small parties. However, unlike Droop's quota, the Hare quota does not guarantee a party with a majority of votes in the district will win at least half the seats.