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Instant-runoff voting (IRV), also known as ranked-choice voting or the alternative vote (AV),^[1] combines ranked voting (in which voters rank candidates rather than choosing only a single preferred candidate) together with a system for choosing winners from these rankings by repeatedly eliminating the candidate with the fewest first-place votes and reassigning their votes until only one candidate is left.^[2][3] It can be seen as a modified form of a runoff election or exhaustive ballot in which, after eliminating some candidates, the choice among the rest is made from already-given voter rankings rather than from a separate election. Many sources conflate this system of choosing winners with ranked-choice voting more generally, for which several other systems of choosing winners have also been used.

IRV is used in national elections in several countries. In Australia, it is used to elect members of the federal House of Representatives,^[4] as well as the lower houses in most states, and in some local government elections. It is used to elect the president of India, the president of Ireland,^[5] and the National Parliament of Papua New Guinea,^[6] as well as to determine the Academy Award for Best Picture^[7] and the Hugo Awards.^[8]

Election procedure

Process

In instant-runoff voting, as with other ranked election methods, each voter orders candidates from first to last. On their ballot, voters mark a "1" beside their first-round vote; a "2" beside their alternative, who receives their vote if the first candidate is eliminated; a "3" beside their next alternative; and so on, until every candidate has been ranked.^[9]

The instant-runoff procedure is as follows:

1. Eliminate the candidate with the fewest votes.
2. If only one candidate remains, elect this candidate and stop.
3. Otherwise, re-assign ballots held by the eliminated candidate to the next available choice. Return to step 1.

Ballots assigned to eliminated candidates are added to the totals of one of the remaining candidates based on the next preference ranked on each ballot.

Optimized procedure

Sometimes, this algorithm is described as electing a candidate as soon as the number of ballots they hold is equal to more than half the total. This is because as soon as a candidate holds over half the votes, it is no longer mathematically possible for another candidate to defeat them (see Droop quota).

A similar observation allows for a more efficient algorithm that can eliminate candidates in batches:

1. If the bottom k candidates, taken together, have less votes than the k+1^th candidate, eliminate the bottom k candidates.
2. If only one candidate remains, elect this candidate and stop.
3. Otherwise, re-assign ballots for eliminated candidate to the next choice. Return to step 1.

This rule does not change the outcome: if the bottom k candidates do not have enough votes, taken together, to catch up to the k+1^th candidate, this means their defeat is mathematically inevitable.

Confusion with majority support

IRV is often mistakenly referred to as electing candidates "with a majority", because the optimized procedure above will stop as soon as any candidate's score exceeds half their remaining votes. However, this is an implementation detail and does not mean candidates in IRV are actually supported by a majority of voters. This can be most clearly illustrated by noting that if not terminated early, the algorithm will ultimately reassign all active votes to the winner, creating an apparent "unanimous winner" (a clear impossibility). In fact, is possible for a candidate to win an instant-runoff election despite no support (i.e. complete opposition) from more than half of all voters; it does so even if there exists a candidate who would be approved of by a majority of the electorate, a trait it shares with first-preference plurality. This occurs when some voters truncate their ballots to indicate they refuse to support any candidate in the final round.^[10] In practice, over half of all instant-runoff elections do not elect a majority-approved candidate because of this effect.^[11]

Social choice theorists generally define a majority-preferred candidate or majority winner as one who would defeat any one of their opponents in a one-on-one majority vote. Instant-runoff voting does not always elect the majority-preferred candidate in this sense either (see #Examples).

Terminology

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 voters do not need to turn out several times to vote.^[12] It is also known as the alternative vote, transferable vote, ranked-choice voting (RCV), single-seat ranked-choice voting, or preferential voting.^[13]

Britons and New Zealanders generally call IRV the "alternative vote" (AV).^[14][15] Australians, who use IRV for most single winner elections, call IRV "preferential voting".^[16] While this term is widely used by Australians, it is somewhat of a misnomer: Depending on how "preferential" is defined, the term would either include all voting systems or else would exclude IRV (as it fails positive responsiveness, implying ballot markings cannot be reinterpreted as "preferences" in the traditional sense).

Jurisdictions in the United States such as San Francisco, Minneapolis, Maine, and Alaska have tended to use the term "ranked-choice voting" in their laws. The San Francisco Department of Elections claimed the word "instant" in the term "instant-runoff voting" could confuse voters into expecting results to be immediately available.^[17][18] As a result of American influence, the term ranked-choice voting is often used in Canada as well.^[19] American NGO FairVote has promoted the terminology "ranked-choice voting" to refer to IRV,^[19][20] a choice that has caused controversy and accusations that the organization is attempting to obscure the existence of other ranked-choice methods that could compete with IRV.^{[citation needed]}

IRV is occasionally referred to as Hare's method^[21] (after Thomas Hare) to differentiate it from other ranked-choice voting methods such as majority-choice voting, Borda, and Bucklin.

When the single transferable vote (STV) method is applied to a single-winner election, it becomes IRV; the government of Ireland has called IRV "proportional representation" based on the fact that the same ballot form is used to elect its president by IRV and parliamentary seats by proportional representation (STV), but IRV is a non-proportional winner-take-all (single-winner) election method, while STV elects multiple winners.^[22] State law in South Carolina^[23] and Arkansas^[24] use "instant runoff" to describe the practice of having certain categories of absentee voters cast ranked-choice ballots before the first round of an election and counting those ballots in any subsequent runoff elections. Zdroj:https://en.wikipedia.org?pojem=Instant-runoff_voting
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