Difference between revisions of "Instant-runoff voting"

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[[Category:Terms and definitions]]
[[Category:Terms and definitions]]

Revision as of 21:02, 29 April 2014

Instant-runoff voting (or IRV) is a voting system used for single winner elections in which voters rank candidates in order of preference.

In an IRV election, if no candidate receives an overall majority of first preferences, the candidates with fewest votes are eliminated one by one, and their votes transferred according to their second and third preferences (and so on) and all votes retallied, until one candidate achieves a majority. The term 'instant runoff voting' is used because this process resembles a series of runoff elections.

At a national level IRV is used to elect the Australian House of Representatives, the President of Ireland, the national parliament of Papua New Guinea and the Fijian House of Representatives. In the United States, it has been adopted in nine local jurisdictions, including three large population cities and counties during the 2006 United States general elections. In the United Kingdom, a form of IRV is used to elect the mayor of London.


IRV example ballot

Instant runoff voting has been called a number of other names. In the United Kingdom, it is known as the Alternative Vote (AV), or as the Alternative Transferrable Vote (ATV). In Australia, the generic term preferential voting is used (though all voting systems aim to discover preference, simple majority voting does not allow expressions of preference through ranking, as does IRV). When used in Canada in the past it was known as the preferential ballot. It is also sometimes known in the United States as ranked choice voting.

When the single transferable vote (STV) system is applied to a single-winner election it becomes the same as IRV. For this reason IRV is sometimes considered to be merely a special form of STV. However, because STV was designed for multi-seat constituencies, many scholars consider it to be a separate system from IRV, and that is the convention followed in this article.

IRV is usually known simply as "STV" in New Zealand and Ireland, although the term the 'Alternative Vote' is also sometimes used in those countries. IRV is sometimes referred to as the Hare system, after Thomas Hare, one of the inventors of STV. It has also been referred to as Ware's method, after its own inventor, William Robert Ware. Writers differ as to whether or not they treat instant runoff voting as a proper noun.

In the United States, especially in California, IRV is called "Ranked Choice Voting."[1]


In IRV the voter ranks the list of candidates in order of preference. Under the most common ballot layout, the voter places a '1' beside their most preferred candidate, a '2' beside their second most preferred, and so forth. In the ballot paper shown at the top-right of this page the preferences of the voter are as follows:

  1. Joe Bobb
  2. Darryl Zero
  3. Righty Fahr

Each voter may be said to cast only one vote but, during the process of counting votes, his vote may be 'transferred' from one candidate to another.

Counting the votes

In an IRV election, every voter has one vote -- at least in the sense that each voter uses only one ballot -- but can express more than one preference. Ballots are initially sorted according to their expressed first preferences. If no candidate achieves an overall majority of first preferences (more than half of the total vote) then the candidate with the fewest first preferences is eliminated. That candidate's votes are recounted and are distributed to the remaining candidates according to the second preferences expressed by each ballot paper. If there is still no candidate with an overall majority of votes then the candidate with the fewest votes in this round of counting is eliminated and his or her votes transferred in the same way, according to the second or third preferences expressed by each ballot paper. This process of counting and eliminating continues until a candidate has obtained an overall majority of votes in the final round of counting.

Once candidates are eliminated or 'excluded', no votes can be transferred to them. Therefore if a ballot paper being recounted expresses a preference for a candidate who has already been excluded, the next 'live' preference on the ballot is used instead.

IRV with batch elimination descibes a two-round method of IRV where if no candidate receives a majority of the first round preferences, all candidates but the top two are eliminated and all ballots are counted for whichever of the two runoff candidates is ranked first on that ballot.

External links

--- Portions of this article were adapted from the article about Instant-runoff voting on Wikipedia, the free encyclopedia. Wikipedia's Copyright Notice can be found here.


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