Supplementary voting system

The contingent vote is an electoral system used to elect a single winner, in which the voter ranks the candidates in order of preference. In an election, if no candidate receives an absolute majority of first preference votes, then all but the two leading candidates are eliminated and there is a second count. In the second count, the votes of those who supported eliminated candidates are distributed among the two remaining candidates, so that one candidate achieves an absolute majority. The contingent vote is similar to the alternative vote but differs from it in that the alternative vote typically allows for many rounds of counting, whereas under the contingent vote there are never more than two. In the United States both the contingent vote and the alternative vote are often referred to as variants of instant-runoff voting. The contingent vote can also be considered a compressed form of the two round system (runoff system), in which both 'rounds' occur without the need for voters to go to the polls twice.

Today, a special variant of the contingent vote is used to elect the President of Sri Lanka. Another variant, called the supplementary vote, is used to elect mayors and Police and Crime Commissioners in England. In the past the ordinary form of the contingent vote was used to elect the Legislative Assembly of Queensland from 1892 to 1942. To date, this has been the longest continuous use of the system anywhere in the world. It was also used in the U.S. state of Alabama in the 1920s.

In an election held using the contingent vote the voters rank the list of candidates in order of preference. Under the most common ballot layout, they place a '1' beside their most preferred candidate, a '2' beside their second most preferred, and so on. In this respect the contingent vote is the same as instant-runoff voting.

There are then a maximum of two rounds of counting. In the first round only first preferences are counted. If a candidate has received an absolute majority of first preferences (i.e. more than half) then he or she is immediately declared the winner. However, if no candidate has an absolute majority then all but the two candidates with the most first preferences are eliminated, and there is a second round. In the second round the votes of the voters whose first preference had been eliminated are transferred to whichever of the two remaining candidates were ranked the highest. The votes are then counted and whichever candidate has an absolute majority is declared elected.

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