Single transferable vote (STV)

In a district electing several members, voters rank candidates. A candidate who reaches a quota is elected; their surplus votes, and the votes of eliminated candidates, pass to next preferences. The result is approximately proportional and candidate-centred rather than party-list based.

Thomas Hare set out the scheme in 1859 and John Stuart Mill endorsed it in Considerations on Representative Government (1861). Proportionality depends on district magnitude; comparative work finds that small multi-member districts can combine proportional outcomes with local accountability.

• Ireland: Article 16 requires Dáil members to be elected by PR-STV in constituencies of at least three seats. Referendums in 1959 and 1968 to replace it with single-member voting failed.
• Australia: the Senate has used PR-STV since the 1949 election.
• Malta, Northern Ireland and Scottish local councils also use STV.

Counting is complex, ballots can be long, and candidates of the same party compete with each other, which critics link to constituency-service politics. Like instant-runoff, STV can be non-monotonic and, in small districts, favours larger parties.

STV has been stable in Ireland, where voters twice kept it. Its effects on party systems depend heavily on district size.