The supplementary vote and the
Sri Lankan contingent vote are two implementation variations, in which voters cannot rank all of the candidates but rather are only permitted to express two or three preferences, respectively. This means that if a voter's marked preferences do not include either of the candidates who survive to the second round, then it will be impossible to transfer the vote, which is therefore declared "wasted" or "exhausted".
Sri Lankan contingent vote In Sri Lanka, since the
1982 presidential election, a variant of the contingent vote
electoral system is used to elect the
country's president. As under the conventional contingent vote, in an election held using the Sri Lankan form of the contingent vote each voter ranks the candidates in order of preference, and if no candidate receives an overall majority of first preference 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. However, whereas under the ordinary form of the contingent vote voters can rank all of the candidates in order of preference, under Sri Lankan contingent voting the voter can only express their top three preferences (which can lead to
exhausted ballots). Each direct presidential election going back to the first in 1981 had seen a candidate from one of the two major parties or alliances at the time winning in the first count so the second round of vote counting had never been conducted. However, in the
2024 presidential election, the strong third-party candidacy of
Anura Kumara Dissanayake, who won 42% of the first preference vote, forced a second round of counting for the first time in the nation's history. Dissanayake went on to win the election in the second round.
Supplementary vote The supplementary vote (SV) is a variation of the contingent vote in which the voter ranks only two of the candidates in order of preference. If a voter's first-choice candidate is eliminated but their second choice is one of the two remaining candidates, their vote is transferred to the second-choice candidate. This means that the winning candidate has the support of a majority of voters who expressed a preference among the top two, although not necessarily a majority of votes cast in the first count. The supplementary vote was used in all elections for
directly elected mayors in England, including the
Mayor of London, and in elections for
police and crime commissioners, until 2022, when it was replaced by
first-past-the-post voting (FPTP).
History and use In the early 1990s, the
Plant Commission was established by the
Labour Party to recommend a new voting system for the
Parliament of the United Kingdom. When the Commission reported in 1993, instead of suggesting an already existing system, it recommended the supplementary vote system, which it said had never been used anywhere. In actuality, contingent voting had been used in Australia as early as 1892. In 2000, several districts in England introduced directly elected mayors. It was decided to use the supplementary vote for the election of these new mayors, including the Mayor of London, and for the election of
police and crime commissioners across much of England and Wales. The supplementary vote was used for these offices from 2000 to 2022. In the 2021 London election, a record 5 percent of ballots were wholly rejected, and no candidate achieved a majority of the vote. The government responded by ending the use of the supplementary vote in 2022, citing voter confusion with a complex system. However, critics, including the Labour and Green parties, argued that the wasted votes were due to ballot layout and that the change was aimed at benefitting Conservative Party candidates. They also claimed that the supplementary vote was effective in increasing multi-party participation and was popular among voters. In July 2025, the British government published the
English Devolution and Community Empowerment Bill, which if passed will revert mayoral elections, along with police and crime commissioner elections, back to the supplementary voting election system. The histories of
two-round voting and other forms of
instant run-off voting may be seen as part of the history of SV due to their similarities.
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 for whom, under systems such as first-past-the-post, they would be discouraged from doing so for tactical reasons. These positive effects are moderated by the incentives SV creates for voting, in some circumstances, for only candidates from among the leading three. Political scientists
Colin Rallings and
Michael Thrasher noted two flaws of SV: • First, since the automatic dual-ballot nature of SV dispenses with any need for a runoff two weeks later – as often happens for, say, the election of the president of France – voters cast their second preferences without being certain of which candidates will make the runoff. Consequently, some second preferences will be declared invalid because they bear only preferences marked for eliminated candidates. • Second, it is possible for the victor to fail to achieve an absolute majority overall, for it is not an obligation for a voter to cast a second preference, and even when a second preference is marked, the vote will be ineffective if it is cast for a candidate who does not make it into the top two, when the first preference is marked also for a candidate who does not make it into the second round. ==Similar systems==