The Supplementary Vote

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Sample ballot paper

The Supplementary Vote (SV) is an electoral system used for the election of a single candidate. It is a variant of the Contingent Vote. Under the Supplementary Vote voters express a first and second choice of candidate only, and if no candidate receives an absolute majority of first choice votes on the first count then all but the two leading candidates are eliminated and their votes redistributed to help determine a winner in a second and final round. The Supplementary Vote is currently used in all elections for directly elected mayors in England, including the Mayor of London.

Voting and counting

Each voter ranks at least one and no more than two candidates by placing an 'X' in one column to indicate his or her first choice of candidate and another 'X' in a second column to indicate his or her second choice of candidate. In the first round of counting only first preferences are tallied. If any candidate has an absolute majority of votes (i.e. more than half) at this stage then are declared elected. If no candidate has a majority then all candidates except the two with most first preferences are eliminated and the count proceeds to a second round. In the second round any voter whose first preference has been eliminated has his vote transferred to the candidate of his second preference (but only if his second choice has not also been eliminated). The candidate with the most votes is then declared elected.

Example

Imagine an election in which there are three candidates: Andrew, Brian and Catherine. There are 100 voters and they vote as follows:

#	36 voters	16 voters	48 voters
1st	Andrew	Brian	Catherine
2nd	Brian	Andrew	Brian

1. To begin the count first preferences are counted, and the tallies stand at:

• Andrew: 36
• Brian: 16
• Catherine: 48

2. No candidate has an absolute majority of votes (this would be 51), so the two candidates with most votes proceed to a second round and Brian, who has the fewest votes, is excluded. All of Brian's supporters have given Andrew as their second preference, so his votes all transfer to Andrew. The tallies then become:

• Andrew: 52
• Catherine: 48

Result: Andrew has the most votes so is declared the winner.

Comparison with similar systems

As noted above, the Supplementary Vote is a variant of the Contingent Vote system, but under SV voters are only permitted to rank two candidates, whereas under the Contingent Vote they can rank all of them. Both SV and the conventional Contingent Vote also differ from instant run-off voting (IRV). In IRV rather than only two rounds there are many; only one candidate is eliminated after each round, and as many rounds occur as are necessary to produce a winner. These differences mean that SV, the conventional Contingent Vote, and IRV can all produce different results.

Under the Supplementary Vote restricting a voter to two preferences means that it might not be possible to transfer her vote to any candidate in the final round–if this occurs her vote is said to be 'exhausted'. If voters are permitted to rank all of the candidates then this cannot occur and every voter will have a chance to contribute to electing a winner in the final round. Because of this SV and the conventional Contingent Vote often produce a different winner. Because under the Contingent Vote all but two candidates are eliminated in the first round it is possible for a candidate to be eliminated who would have gone on to win had he been allowed to receive transfers in later rounds. This fact differentiates all forms of the Contingent Vote from IRV.

Example

Imagine an election in which there are four candidates: Andrea, Brad and Carter and Delilah. There are 100 voters. If they were asked to rank all of the candidates in order of preference then their preferences would be as follows:

#	47 voters	20 voters	18 voters	15 voters
1st	Andrea	Brad	Carter	Delilah
2nd	Brad	Carter	Brad	Carter
3rd	Carter	Delilah	Delilah	Brad

However under the Supplementary Vote the voters are only permit to express their top two preferences, so they vote as follows:

#	47 voters	20 voters	18 voters	15 voters
1st	Andrea	Brad	Carter	Delilah
2nd	Brad	Carter	Brad	Carter

1. To begin the count first preference votes would be tallied, and found to be as follows:

• Andrea: 47
• Brad: 20
• Carter: 18
• Delilah: 15

2. No candidate has an absolute majority so Andrea and Brad would proceed to the second round, while the other two would be excluded. Carter supporters all have Brad as their second choice so their votes would now transfer to Brad. However Delilah voters have Carter as their second choice so, because he is no longer in the count, their votes cannot be transferred to anyone. After the second count the results are therefore:

• Andrea: 47
• Brad: 38

Result: Andrea has the most votes so is the winner.

In the example the result is effected by the fact that Delilah supporters are only allowed to express two preferences, and therefore their votes cannot be transferred to either of the two candidates in the last round. The third preference of Delilah supporters was for Brad. If this preferences had been taken into account then Delilah's votes would have transferred to Brad in the final round and Brad would have been the winner instead of Andrea. For the reason given if the conventional form of the Contingent Vote had been used in the example instead of SV then Brad would have been elected.

The example is also influenced by the fact that their are only two rounds. If IRV were used in the example, instead of either form of the Contingent Vote, then Carter would have been elected. This is because after the first round only Delilah would have been eliminated. His votes would then transfer to Carter and the tallies would become:

• Andrea: 47
• Brad: 20
• Carter: 33

Brad would then be eliminated and his votes would transfer to Carter, who would then have an absolute majority of votes.

History and current use

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Ken Livingstone, the first directly elected Mayor of London. In England elected mayors are chosen by means of the Supplementary Vote.

The Supplementary Vote has been used since 2000 to elect the Mayor of London. At the start of the twenty-first century it was also in use for the direct election of eleven English mayors. The system was invented in 1993 by the Plant Commission. This was a commission established by the Labour Party, which was then in opposition, to recommend a new voting system for the British Parliament. Instead of suggesting an already existing system the Commission chose to devise its own. However it never became official Labour Party policy to introduce the Supplementary Vote for national elections in the United Kingdom. Prior to 2000 there were no directly elected mayors in England. When direct elections were introduced for some mayors it was decided to use SV.

A system similar to the Supplementary Vote has been used to elect the President of Sri Lanka since 1982. Under the Sri Lankan system voters are permitted to express, from among the list of candidates, not just a first and second but also a third preference.

Potential for tactical voting

The Supplementary Vote is open to the same forms of tactical voting as other forms of the Contingent Vote. These include the tactics of 'compromise' and 'push over'. However the fact that voters are only permitted to express two preferences makes it especially vulnerable to 'compromise'. This is because it makes it necessary for voters to express either a first or a second preference for one of the two leadings candidates if they wish to influence the result of the election. If lower preferences were allowed then the voter would, in many circumstances, be able to give her first two preferences to her two favourite candidates, even if neither was likely to be elected; if both had only minor support then the voter could still influence the election by using her third or subsequent preferences to support more popular candidates.

Impact on factions and candidates

The Supplementary Vote is said to encourage candidates to seek support beyond their core base of supporters in order to secure the second preferences of the supporters of other candidates, and so to create a more conciliatory campaigning style among candidates with similar policy platforms. SV is also likely to improve the chances of 'third party' candidates by encouraging voters who wish to do so to vote sincerely for such candidates where under systems such as 'first past the post' they would be discouraged from doing so for tactical reasons.

These potentially positive effects will be moderated, however, by the strong incentives the Supplementary Vote creates for voting, in most circumstances, only for candidates from among the leading three. The Electoral Reform Society criticised the Supplementary Vote following the mayoral election in Torbay in October 2005, claiming that 43.5% of second preference votes were ignored as not being given to either of the top two placed candidates, disadvantaging supporters of non-party candidates^[1].

Like other forms of the Contingent Vote, the Supplementary Vote is not a form of proportional representation and so, were it used to elect a council or legislature, it could be expected to produce results similar to other systems involving single seat constituencies such as the 'first past the post' (plurality) system.

Note

1. ^ See Electoral Reform Society press release on Torbay election

See also

• Mayors in the United Kingdom

External links

• LondonElects: How the Mayor of London is Elected
• Electoral Systems Index: Sri Lanka