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The Single Transferable Vote, or STV, is a voting system designed to accurately achieve proportional representation in multi-candidate elections. When applied in a proportional representation setting in multi-candidate elections, it is generally known as Proportional Representation through the Single Transferable Vote or PR-STV. When similar methods are applied to single-candidate elections they are sometimes called instant-runoff voting or the alternative vote and have different implications for a similar ballot. In both systems of voting the ballot choices represent an ordinal ranking of preferences, but they are tallied differently, since an "instant runoff" for only one position or measure is a trivial calculation.
Places that use STV for governmental elections include:
STV enjoyed some popularity in the United States in the first half of the 20th Century. The community school boards of the City of New York used STV until they were abolished in 2002.
The method used for electing the Legislative Assemblies of Tasmania and the Australian Capital Territory is called the "Hare system" or the "Hare-Clark system" after Thomas Hare, an English solicitor who developed the system, and Andrew Inglis Clark, a Tasmanian Attorney-General who introduced STV into State law. STV was used for provincial elections in the province of Alberta, Canada from 1926 to 1955.
Each voter ranks all candidates in order of preference. For example:
When all the votes have been cast, a winning quota is set. The most common formula for the quota is the Droop Quota which is most often given as:
Other quotas used include the 1981 election in Malta. In this election the Maltese Labour Party won a majority of seats despite the Nationalist Party winning a majority of first preference votes. This caused a constitutional crisis, leading to provision for the possibility of bonus seats. These bonus seats were used in 1987 and again in 1996. Similarly, the Northern Ireland elections in 1998 led to the Ulster Unionists winning more seats than the Social Democratic and Labour Party, despite winning a smaller share of the vote.
The single transferable vote eliminates much of the reason for tactical voting. Voters are "safe" voting for a candidate they fear won't be elected, because their votes will be reallocated in Process B. They are "safe" voting for a candidate they believe will receive overwhelming support, because their votes will get reallocated in Process A.
However, in older STV systems there is a loophole: candidates who have already been elected do not receive any more votes, so there is incentive to avoid voting for your top-ranked candidate until after they have already been elected. For example, a voter might make a tactical decision to rank their top-place candidate beneath a candidate they know will lose (perhaps a fictional candidate). If the voter's true top-place candidate has not been elected by the time their fake top candidate loses, the voter's full vote will count for their true top-place candidate. Otherwise, the voter will have avoided having had their ballot in the lottery to be "wasted" on their top-ranked candidate, and will continue on to lower-ranked candidates.
Note that in more modern STV systems, this loophole has been fixed. A vote receives the same fractional weighting regardless of when it arrives at the successful candidate.
There are also tactical consideration for parties standing more than one candidate in the election. Standing too many candidates might result in first-preference votes being spread amongst them, and several being eliminated before any are elected and their second-preference votes distributed. Standing too few may result in all the candidates being elected in the early stages, and votes being transferred to candidates of other parties.
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