24 Facts About Instant-runoff voting

Instant-runoff voting is a type of ranked preferential voting method.

IRV is usually used where first-past-the-post Instant-runoff voting was abandoned, and to be better in line with the majority rule in single-winner elections.

Where preferential Instant-runoff voting is used for the election of an assembly or council, parties and candidates often advise their supporters on their lower preferences, especially in Australia where a voter must rank all candidates to cast a valid ballot.

All forms of ranked choice Instant-runoff voting reduce to plurality when all ballots rank only one candidate.

Since Australia has compulsory Instant-runoff voting it is difficult to tell how many ballots are deliberately spoiled.

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Instant-runoff voting derives its name from the way the ballot count simulates a series of runoffs, similar to an exhaustive ballot system, except that voter preferences do not change between rounds.

American NGO FairVote uses the terminology "ranked choice Instant-runoff voting" to refer to IRV in the case of single-winner offices and to refer to single transferable vote in the case of multi-winner offices.

Instant-runoff voting has notably high resistance to tactical voting but less to strategic nomination.

Gibbard–Satterthwaite theorem demonstrates that no Instant-runoff voting method using only the preference rankings of the voters can be entirely immune from tactical Instant-runoff voting.

The argument that IRV represents plural Instant-runoff voting is sometimes used in arguments over the "fairness" of the method, and has led to several legal challenges in the United States.

Term preferential Instant-runoff voting refers to any of a number of Instant-runoff voting methods by which, on a single ballot when there are more than two possible choices, the second or less-preferred choices of voters can be taken into account if no candidate or proposition attains a majority.

System of preferential Instant-runoff voting just described should not be used in cases where it is possible to follow the normal procedure of repeated balloting until one candidate or proposition attains a majority.

Two other books on American parliamentary procedure take a similar stance, disapproving of plurality Instant-runoff voting and describing preferential Instant-runoff voting as an option, if authorized in the bylaws, when repeated balloting is impractical: The Standard Code of Parliamentary Procedure and Riddick's Rules of Procedure.

In runoff Instant-runoff voting voters do not rank candidates in order of preference on a single ballot.

All multi-round runoff Instant-runoff voting methods allow voters to change their preferences in each round, incorporating the results of the prior round to influence their decision.

Simplest form of runoff Instant-runoff voting is the two-round system, which typically excludes all but two candidates after the first round, rather than gradually eliminating candidates over a series of rounds.

Conversely, a practical benefit of 'contingent Instant-runoff voting' is expediency and confidence in the result with only two rounds.

Optional preferential Instant-runoff voting is used for elections for the President of Ireland and the New South Wales Legislative Assembly.

Full preferential Instant-runoff voting is used for elections to the Australian federal parliament and for most State parliaments.

Version of instant-runoff voting applying to the ranking of parties was first proposed for elections in Germany in 2013 as spare vote.

Scholars rate Instant-runoff voting methods using mathematically derived Instant-runoff voting method criteria, which describe desirable features of a method.

Many of the mathematical criteria by which Instant-runoff voting methods are compared were formulated for voters with ordinal preferences.

Result of the 1990 Irish presidential election provides an example of how instant-runoff voting can produce a different result from first-past-the-post voting.

Real-life example of IRV producing a result which differs from what would be expected under a first-past-the-post or the two-round Instant-runoff voting system is the result for the seat of Prahran in the 2014 Victorian state election.