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Instant-runoff voting
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Instant-runoff voting

When the Single Transferable Vote voting system is applied to a single-winner election it is sometimes called Instant-runoff voting (IRV), as it is much like holding a series of runoff elections in which the lowest polling candidate is eliminated in each round. IRV is often considered independently of multi-winner STV because it is simpler and because it is the most widely advocated electoral reform in the USA.

Instant-runoff voting is also known as Ranked Choice Voting (RCV), a term useful for describing the voter's experience as well as the appearance of the ballot.

Outside the USA, IRV is referred to as single-winner STV, the Hare System, Alternative Vote, Preference Vote or preferential voting, though there is room for confusion with some of these terms since they can also refer to STV in general.

Single winner STV is used to elect the Australian House of Representatives, the lower houses of most of Australia's state parliaments, the President of Ireland, the Fijian House of Representatives, and the Parliament of Nauru. See below for a more detailed list.

Table of contents
1 Voting
2 Counting the votes
3 Potential for tactical voting
4 Effect on parties and candidates
5 Where IRV is used
6 Adoption in the United States
7 Special cases of IRV eliminations
8 See also:
9 External links


Each voter ranks at least one candidate in order of preference. In most Australian elections, voters are required to rank all candidates. In other elections, votes may be "truncated", for example if the voter only ranks his first five choices.

Counting the votes

First choices are tallied. If no candidate has the support of a majority of voters, the candidate with the least support is eliminated. A second round of counting takes place, with the votes of supporters of the eliminated candidate now counting for their second choice candidate. After a candidate is eliminated, he or she may not receive any more votes.

This process of counting and eliminating is repeated until one candidate has over half the votes. This is equivalent to continuing until there is only one candidate left.

An example

Imagine an election for the capital of Tennessee, a state in the United States that is over 500 miles east-to-west, and only 110 miles north-to-south. In this vote, the candidates for the capital are Memphis, Nashville, Chattanooga, and Knoxville. The population breakdown by metro area is as follows:

If the voters cast their ballot based strictly on geographic proximity, the voters' sincere preferences might be as follows:

42% of voters (close to Memphis)
  1. Memphis
  2. Nashville
  3. Chattanooga
  4. Knoxville
26% of voters (close to Nashville)
  1. Nashville
  2. Chattanooga
  3. Knoxville
  4. Memphis
15% of voters (close to Chattanooga)
  1. Chattanooga
  2. Knoxville
  3. Nashville
  4. Memphis
17% of voters (close to Knoxville)
  1. Knoxville
  2. Chattanooga
  3. Nashville
  4. Memphis

City Round 1 Round 2 Round 3
Memphis 42 42 42
Nashville 26 26 26 0
Chattanooga 15 15 0 0
Knoxville 17 17 32 32 58

Chattanooga, having the smallest vote, is eliminated in the first round. All of the votes for Chattanooga have Knoxville as a second choice, so they are transferred to Knoxville. Nashville now has the smallest vote, so it is eliminated. The votes for Nashville have Chattanooga as a second choice, but as Chattanooga has been eliminated, they instead transfer to their third choice, Knoxville. Knoxville now has 58% of the vote, and it is the winner.

In a real election, this election result is unlikely, since it would be extreme to imagine that all of Chattanooga's voters would rank Knoxville second.

Potential for tactical voting

All reasonable voting systems allow for some form of tactical voting, also known as strategic voting. Unlike some systems, though, instant-runoff voting does not satisfy the monotonicity criterion—in some situations, if a voter or group of voters decides to rank a preferred candidate \lower, it can result in that candidate winning the election, whereas if they had ranked the candidate higher, according to their sincere preference, that candidate would not have won. Some other election methods, including traditional plurality elections (in which the candidate who gets the most votes wins, even if they did not earn a majority), do not have this particular drawback.

In addition, voters in an instant-runoff election may use more familiar tactical voting techniques known as "compromise" and "burying", in which they would lower their ranking of a preferred candidate to increase the likelihood of a more preferred outcome. Traditional plurality and runoff elections are also vulnerable to these forms of tactical voting.

In the above example for the case of Tennessee, if the voters from Memphis are aware that they do not comprise half of the voters, and that Memphis is the last choice of all other voters, they can "compromise" by ranking Nashville over Memphis, thus ensuring that Nashville, their second choice, will win, rather than Knoxville, their last choice.

Alternatively, if voters from Memphis are unlikely to vote tactically (because they think they have a chance of winning outright or for any other reasons), voters from Nashville may improve their result by "compromising" and ranking Chattanooga over Nashville. This would allow Chattanooga to defeat Knoxville in the first round and go on to become eventual winner, a better result for Nashville voters than a Knoxville win.

Finally, voters may also engage in another type of strategic voting, by intentionally promoting "push-overs", candidates they know are unlikely to win (aka "burying" the others). This can benefit voters by bringing their preferred candidate to a more winnable final runoff round, basically using the pushovers as a shield for protection of their other vote.

Effect on parties and candidates

Unlike runoff voting, however, there are no chances to deal in between rounds, change voters' minds, or gain support of the other candidates.

Giving them only one chance to do so, instant runoff preference voting encourages candidates to balance earning core support through winning first choice support and earning broad support through winning the second and third preferences of other candidates' core supporters. As with any winner-take-all voting system, however, any bloc of more than half the voters can elect a candidate regardless of the opinion of the rest of the voters.

This is considered a weakness by the advocates of a more deliberative democracy, who point to the French system of presidential election where such between-round dealings are heavily exploited and useful (they say) to draw together a very factionalized electorate.

Advantages to rank preference ballot: (IRV)

Advantages to sequential balloting: (runoff voting)

Where IRV is used

The single-winner variant of STV is used in Australia for elections to the Federal House of Representatives, for the Legislative Assemblies ("lower houses") of all states and territories except Tasmania and the Australian Capital Territory, which use regional and statewide multi-member constituencies respectively. It is also used for the Legislative Councils of Tasmania and Victoria, although the latter will switch to the multi-member variant from 2006. The Australian Senate is elected by the multi-member variant of STV.

Ireland uses STV to elect its own parliament and its delegation to the European Parliament (by the multi-member variant), and its President (by the single-member variant). Northern Ireland also uses the multi-member variant for elections to its Assembly and for its European Parliamentary MPs. Malta uses the multi-member variant for its parliamentary elections.

In the Pacific, the STV system is used to elect the parliament of Nauru and the Fijian House of Representatives. Papua New Guinea has also decided to adopt it for future elections, starting in 2007. The Fijian system has been modified to allow for both "default preferences", specified by the political party or candidate, and "custom preferences", specified by the voter. Each political party or candidate ranks all other candidates according to its own preference; voters who are happy with that need only to vote for their own preferred candidate, whose preferences will automatically be transferred according to the ranking specified by the candidate. Voters who disagree with the ranking, however, may opt to rank the candidates according to their own preferences. In the last election, about a third of all voters did so.

Some or all local body elections in the above mentioned countries and others (New Zealand) use IRV to elect mayors. Political parties, cooperatives and other private groups also use STV and/or IRV.

See Table of voting systems by nation

Adoption in the United States

Suggested by Robert's Rules of Order, instant-runoff voting is increasingly used in the United States for non-governmental elections, including student elections at many major universities.

Notable supporters include Republican U.S. Senator John McCain and 2004 Democratic presidential primary election candidates Howard Dean and Dennis Kucinich. The system is favored by many third parties, most notably the Green Party and the Libertarian Party, as a solution to the "spoiler" effect third-party sympathizers suffer from under plurality voting (i.e., voters are forced to vote tactically to defeat the candidate they most dislike, rather than for their own preferred candidate). In order to increase awareness of the voting method and to demonstrate it in a real-world situation, the Independence Party of Minnesota tested IRV by using it in a straw poll during the 2004 Minnesota caucuses (results favored John Edwards).

This dilemma rose to attention in the United States in the 2000 election. Supporters of Ralph Nader who nevertheless preferred Democrat Al Gore to Republican George W. Bush found themselves caught in a dilemma. They could vote for Nader, and risk Gore losing to Bush, or, they could vote for Gore, just to make sure that Bush is defeated.

In March 2002, an initiative backed by the Center for Voting and Democracy passed by referendum making instant runoff voting the means of electing local candidates in San Francisco. It will be used in that city in Fall of 2004. (Note: The San Francisco Department of Elections prefers the term "Ranked Choice Voting" because "the word 'instant' might create an expectation that final results will be available immediately after the polls close on election night.")

In September 2003, an amendment to the California State Constitution was proposed (SCA 14) with wide-ranging goals of election reform, including ranked-choice voting for statewide offices.

A group in Ferndale, Michigan is attempting to implement IRV for mayoral elections in that Detroit suburb.

Special cases of IRV eliminations

Instant Runoff Voting as an ideal does not explicitly define how to handle special cases such as ties and different rules can be considered. A good IRV election must define rules to handle these cases before the votes are cast. The reason why is that there are cases where one set of rules will select a winner different from another set of rules and the set of rules used may affect how the voters cast their ballots.

Especially when performing IRV counts on smaller elections, there can be frequent last-place ties that prevent clear bottom elimination.

Here are some approaches to consider, individually and combined. The first class of rules allows many candidates to be eliminated at the first count regardless of actual ties. These are practical rules before the first round that reward stronger candidates among the full set of competition. Such rules won't likely affect the winner but they will reduce the number of elimination rounds and thus the number of opportunities for ties to develop. A second class of rules consider actual ties that can't be avoided.

  1. Consider multicandidate elimination of weak candidates as the first step:
    • CANDIDATE COUNT: Define a maximum number of candidates that can survive the first round. (Example top-two)
    • VOTE MINIMUM: Define a minimum vote threshold (5 vote for example) and eliminate all weaker candidates together. (Requires limitations for rule to apply)
    • PERCENT MINIMUM: Define a minimum percent vote threshold (5% for example) and eliminate all weaker candidates together. (Again, requires limitations for application)
    • PERCENT RETENTION: Define a minimum percent of votes by top candidates to be retained. (Example - retain the top set of candidates who combined control 50% of the vote)
  2. Tie-breaking rules:

See also:

External links