Single Transferable Vote
category:Elections\n
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.
Single transferable vote is used, among other places, for all elections in the
Republic of Ireland [1],
Northern Ireland\nand
Malta [1], to elect the
Australian Senate [1] and the City Council in
Cambridge, Massachusetts. \nThe 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.
Voting
Each voter ranks all candidates in order of preference. For example:
- Andrea\n#Carter\n#Brad\n#Delilah
\nSetting the Quota
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:\n:.
\nOther quotas used include the Hare Quota:
-
and the Imperiali Quota:
For those keeping track, the size of the quota is then generally Hare > Droop > Imperiali.
Counting The Votes
Process A: Top-preference votes are tallied. If one or more candidates have received more votes than the quota, they are declared elected. After a candidate is elected, they may not receive any more votes.
The excess votes for the winning candidate are reallocated to the next-highest ranked candidates on the ballots for the elected candidate. There are different methods for determining how to reallocate the votes. Some versions use random selection, others count each ballot fractionally.
Process A is repeated until there are no more candidates who have reached the quota.
Process B: The candidate with the least support is eliminated, and\ntheir votes are reallocated to the next-highest ranked candidates on\nthe eliminated ballots. After a candidate is eliminated, they may not\nreceive any more votes.
After each iteration of Process B is completed, Process A starts\nagain, until all candidates have been elected or eliminated.
An example
2 seats to be filled, four candidates: Andrea, Brad, Carter, and\nDelilah.
5 voters rank the candidates:
- Andrea\n#Brad\n#Carter\n#Delilah
17 voters rank the candidates:
- Andrea\n#Carter\n#Brad\n#Delilah
8 voters rank the candidates:
- Delilah
The threshold is:
\nIn the first round, Andrea receives 22 votes and Delilah 8. Andrea is\nelected with 11 excess votes. Her 11 excess votes are reallocated to their second preferences (which votes are chosen may be decided by random selection). For example, 8 of the reallocated votes are for Carter, 3 for Brad. Note: this is not a realistic example - elections with a small number of votes often have special rules - for example, Irish Senate elections are conducted using thousandths of votes.
As none of the candidates have reached their threshold, Brad, the\ncandidate with the fewest votes, is eliminated. All of his votes have\nCarter as the next-place choice, and are reallocated to Carter. This\ngives Carter 11 votes and he is elected.
Is STV a proportional voting system?
STV is not a proportional system in the strict sense. STV does not guarantee that a party will get the same percentage of seats as it gets as a percentage of votes. In fact the notion of a vote "for a party" is less meaningful for STV because votes are not necessarily for a single party. A vote can list candidates from an assortment of poltical parties, in any order. The candidates that are elected reflect the combined preferences of all votes cast.
Another complication with proportionality under STV is the constituency system, where a set of candidates are elected in each electoral district. There is no explicit process for balancing the votes between constituencies, so the overall electoral result is merely the sum of the constituency results.
Within a constituency, however, STV can be said to be proportional for whatever characteristics the voters valued. For example, a portion of voters ranking all women first will result in this portion rounded down to next quota will of women at least represented.
STV provides this proportionality simply by wasting as few votes as possible. A vote is "wasted" if it does not elect anyone; it is partially wasted if it elects someone who gets more votes than is necessary to be elected. STV transfers votes that would otherwise be wasted, and it only transfers such votes.
The degree of national proportionality achieved is strongly related to the district magnitude, or the number of seats that are to be filled at any election. For example, under the Droop quota in a three-seat district, one vote less than a quarter of the total number of votes may not directly contribute to the election of a representative. Therefore, a desire for a high degree of proportionality is best support by large district magnitude.
The proportionality of STV can be controversial, especially in close elections such as 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
SDLP, despite winning a smaller share of the vote.
Potential for Tactical Voting
The single transferable vote eliminates much of the reason for tactical voting.\nVoters are "safe" voting for a candidate they fear won't be elected, \nbecause their votes will be reallocated in Process B. They are "safe" voting for a candidate they believe will receive overwhelming support, because their votes\nwill get reallocated in Process A.
However, in older STV systems there is a loophole: \ncandidates who have already been elected do not receive any more\nvotes, so there is incentive to avoid voting for your top-ranked\ncandidate until after they have already been elected. For example, a\nvoter might make a tactical decision to rank their top-place candidate\nbeneath a candidate they know will lose (perhaps a fictional\ncandidate). If the voter's true top-place candidate has not been\nelected by the time their fake top candidate loses, the voter's full\nvote will count for their true top-place candidate. Otherwise, the\nvoter will have avoided having had their ballot in the lottery to be\n"wasted" on their top-ranked candidate, and will continue on to\nlower-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.
See also
\n* Table of voting systems by nation\n*
Single Non-Transferable Vote\n*
preference voting\n*
Instant-runoff voting\n*
voting systems\n* Hagenbach-Bischoff quota\n* Hare quota\n* Imperiali quota\n* Ross quota
External link
\n* ODP category\n*
pSTV--software for computing the single transferable vote