Droop quota

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In the study of electoral systems, the Droop quota (sometimes called the Hagenbach-Bischoff, Britton, or Newland-Britton quota^[1][a]) is the minimum number of votes needed for a party or candidate to guarantee a full seat in a legislature.^[2]

The Droop quota generalizes the concept of a majority to multiwinner elections. Just as a candidate with a majority (more than half of all votes) is guaranteed to be declared winner in a one-on-one election, a candidate who holds more than one Droop quota's worth of votes at any point is guaranteed to win a seat in a multiwinner election.^[b]

Besides establishing winners, the Droop quota is used to define the number of excess votes, votes not needed by a candidate who has been declared elected. In proportional quota-rule systems such as STV and CPO-STV, these excess votes can be transferred to other candidates, preventing them from being wasted.

The Droop quota was first devised by the English lawyer and mathematician Henry Richmond Droop (1831–1884), as an alternative to the Hare quota. Hagenbach-Bischoff also wrote on the quota in 1888, in his study entitled Die Frage der Einführung einer Proportionalvertretung statt des absoluten Mehres.

Today, the Droop quota is used in almost all STV elections, including those in the Republic of Ireland, Northern Ireland, Malta, and Australia.^{[citation needed]} It is also used in South Africa to allocate seats by the largest remainder method.^{[citation needed]}

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