Instant-runoff voting

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Instant runoff voting (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 run-off elections. At a national level IRV is used to elect the Australian House of Representatives[1], the President of Ireland, the Fijian House of Representatives and, beginning in 2007, the National Parliament of Papua New Guinea. In the United States, it has been adopted by voters in eight local jurisdictions, starting with San Francisco, and, during the 2006 United States general elections, Pierce County, Washington, Minneapolis, Minnesota and Oakland, California.

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[edit] Terminology

Instant runoff voting has been called a number of other names. It is called instant runoff voting in the United States primarily because of its resemblance to runoff voting, which is also used in that country and many presidential elections around the world. It is known as the Alternative Vote (AV) in the United Kingdom and preferential voting in Australia, but this last term can be misleading because IRV is only one of a number of preferential voting systems. When used in Canada in the past it was known as the preferential ballot. It is also sometimes known, in the U.S, as ranked choice voting.

When the single transferable vote (STV) system using the Droop quota is applied to a single-winner election it becomes the same as IRV, since in using the Droop quota formula i.e. votes/(# seats + 1), increased to the next whole number, the resulting quota is 50% + 1, the minimum number needed to win. 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 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 New Zealand local body elections, STV is always referred to as STV, even if only one seat is to be filled (in the case of the mayor being elected.)

[edit] Voting

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. John Citizen
  2. Mary Hill
  3. Jane Doe

[edit] Counting the votes

In an IRV election every voter has one and only one vote, but may count for a lower-ranked choice after the first round of counting. 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) 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 on each ballot paper. If there is still no candidate with an overall majority of votes then the candidate with the fewest votes is again eliminated and the votes counted for the next ranked choice in the same way, according to the second or third preferences expressed on each ballot paper. This process of counting and eliminating continues until a candidate has obtained a majority of 'continuing ballots' — meaning those ballots expressing preferences among candidates who have not been eliminated.

Once candidates are eliminated or 'excluded', no votes can be transferred to them because they did not have enough core support to make that runoff round of counting. Therefore if ballots being recounted express a preference for a candidate who has already been excluded, their next 'live' preference is used. The count is intended to continue only until one candidate has a majority (50%, plus one) of 'continuing ballots', at which point they cannot be defeated.

[edit] Elimination process ties

Ties can happen in any election, but there are more opportunities for a meaningful tie in a series of instant runoff rounds of counting. If there is a tie for last place in the elimination process, various rules must be offered to break it. Typically, the first step is to see if the tie has any chance of affecting the result. If the total of all the combined votes of any grouping of the candidates with the fewest votes is less than the votes cast for the next weakest candidate, then all those bottom tier candidates can be eliminated simultaneously.

But if the tied candidates have a mathematical chance to win, options for breaking the tie include:

  1. Eliminating the weaker candidate in the tie from the previous round, or looking recursively backwards. (May not help)
  2. Eliminating all candidates in the tie together. (Not applicable for ties above third place)
  3. Eliminating one candidate in the tie randomly. (Problematic on election recounts if ties change.)
    • Alternately (for 3-way ties) eliminating all except one candidate in the tie randomly.

Another approach in some private elections is to conditionally eliminate candidates from the tie and recount to see if either (or any) can survive. Usually the full set will become eliminated in any order. However this option is not allowed in a political election because it would allow some voters to have two simultaneous votes.

[edit] Ballot paper

As seen above, voters in an IRV election rank candidates on a preferential ballot. IRV systems in use in different countries vary both as to ballot design and as to whether or not voters are obliged to provide a full list of preferences. In elections such as those for the President of Ireland and the New South Wales Legislative Assembly, voters are permitted to rank as many or as few candidates as they wish. This is known in Australia as 'optional preferential voting'.

Under optional preferential voting some voters may rank only the candidates of a single party, or of their most preferred parties. A minority of voters, especially if they do not fully understand the system, may 'bullet vote', expressing only a first preference. Allowing voters to rank only as many candidates as they wish grants them greater freedom but can also lead to some voters ranking so few candidates that their vote eventually becomes 'exhausted'–that is, at a certain point during the count it can no longer be transferred and therefore loses an opportunity to influence the result.

A 'how-to-vote' card from the Australian federal election of 2004. Australian parties issue 'how to vote' cards as a way of advising their supporters how to use their lower preferences.
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A 'how-to-vote' card from the Australian federal election of 2004. Australian parties issue 'how to vote' cards as a way of advising their supporters how to use their lower preferences.

To prevent exhausted ballots, some IRV systems require or request that voters to give a complete ordering of all of the candidates in an election - if a voter does not rank all candidates her ballot may be considered spoilt or an informal ballot. In Australia this variant is known as 'full preferential voting'. However, when there is a large set of candidates this requirement may prove burdensome and can lead to "donkey voting" in which, where a voter has no strong opinions about her lower preferences, the voter simply chooses them at random. Partly to overcome these problems, in elections to the Australian House of Representatives many parties distribute 'how-to-vote' cards (right), recommending how to allocate preferences on the ballot paper.

The common way to list candidates on a ballot paper is alphabetically or by random lot, a process whereby the order of the candidates published on the ballot paper is determined by lottery. In some cases candidates may also be grouped by party.

Any fixed ordering of candidates on the ballot paper will give some candidates an unfair advantage, because voters, consciously or otherwise, are influenced in their ordering of candidates by the order on the ballot paper. To help further reduce this problem some systems involve a process of random ordering of candidates lists. Taking this even further is the concept of the Robson Rotation, a system where the order of candidates on the paper is randomly changed for each print run of the same election's ballot papers. This means that any one ballot paper is almost certainly different from the next, and thus nullifies the effect of donkey voting.

[edit] History and current use

Main article: History and use of instant-runoff voting

Instant runoff voting was invented around 1870 by American architect William Robert Ware. He evidently based IRV on the single-winner outcome of the Single Transferable Vote, originally developed by Carl Andrae and Thomas Hare. Today IRV is used in Australia for elections to the Federal House of Representatives, and for the Legislative Assemblies (lower houses) of all states and territories except Tasmania and the Australian Capital Territory, which use STV. It is also used for the Legislative Council of Tasmania. In the Pacific, IRV is used for the Fijian House of Representatives, and Papua New Guinea has decided to adopt it for future elections, starting in 2007. IRV is also used to elect the President of Ireland and for municipal elections in various places in Australia, the United States, and New Zealand.

[edit] United States

  • San Francisco has used instant runoff voting annually to elect its Board of Supervisors and major citywide offices since 2004.
  • Burlington, VT completed its first mayoral election using IRV in 2006 after voters approved it in 2005.
  • Ferndale, MI passed instant runoff voting with 68& in 2004 pending necessary implementation.
  • Berkeley, CA passed instant runoff voting with 72% in 2004 pending necessary implementation.
  • Pierce County, WA passed instant runoff voting in November 2006 [2] for implementation for most of its county offices in 2008.
  • Takoma Park, MD adopted instant runoff voting for city council and mayoral elections in 2006 after an 84% win in a 2005 advisory ballot measure and will hold its first IRV election to fill a city council vacancy in January 2007.
  • Minneapolis, MN [3]in November 2006 passed instant runoff voting with 65%. Implementation is scheduled for the 2009 municipal elections.
  • North Carolina adopted instant runoff voting for certain judicial vacancies and will permit municipal pilot programs starting in 2007.
  • Oakland, CA voters passed a measure by 69% to 31% in November 2006 to adopt IRV for its city offices.
  • Dozens of American colleges and universities [4] use IRV, including as of November 2006 more than half of the 30 universities rated most highly by U.S. News and World Report.

[edit] Application in absent voting

Instant runoff voting and variations have been hailed as a solution to the logistical problems of overseas voting in states with runoff provisions. In the event of a runoff, election administrators would have to print new ballots, mail them to far-flung places, and receive them again. In the short window between the first election and the runoff, there often is not enough time. With a ranked instant runoff ballot, the votes of overseas citizens can count even if their first choice does not make the runoff, all on a single ballot.

Arkansas, Louisiana and South Carolina all use forms of instant runoff voting on ballots for military and overseas voters.

[edit] Practical implications

Instant runoff voting is more complex, both in terms of casting votes and counting them, than simpler systems such as 'first-past-the-post' plurality. Voters must rank candidates in order of preference rather than merely write an 'x' beside a single candidate. Changing from plurality to IRV may therefore require the replacement of voting machinery.

IRV has been implemented in cities using optical scan machines, as in San Francisco (CA) and Burlington (VT). A hand count also is possible under IRV and is the method used in most non-American jurisdictions; however it is usually more time-consuming than a quick plurality count, and may need to occur over a number of rounds. It is nonetheless simpler than the count under some other preferential voting systems. IRV is typically less expensive than runoff voting because it is only necessary for voters to go to the polls once. For this reason it may also be less likely to induce voter fatigue.

Under IRV, unlike some other preferential systems, the record of votes cast in a particular area cannot be conveniently summarised for transfer to a central location in which they can be counted. If areas were to report the number of votes cast for each possible order of candidates, as in the examples above, the permutations can be very large as the number of possible orders is equal to the factorial of the number of candidates. Three candidates would produce only six combinations but five candidates would produce 120 and ten candidates 3.6 million. This unwieldiness could prolong the counting procedure, provide more opportunities for undetected tampering than in more easily summable methods, and make recounts more costly. What happens in practice in Australia is a simplified count is sent through to the central location on the night with the actual ballot papers transported securely to the central location for the final count. In Ireland's presidential race, there are several dozen counting centers around the nation. Each center reports its totals for each candidate and receives instructions from the central office about which candidate or candidates to eliminate in the next round of counting.

[edit] Majoritarianism and consensus

The intention of IRV is that the winning candidate will have the support of an absolute majority of voters, or at the very least an overall majority. It is often intended as an improvement on the 'First Past the Post' (plurality) voting system. Under 'First Past the Post' the candidate with most votes (a plurality) wins, even if they do not have an overall majority (more than half) of votes. IRV tries to overcome this problem by eliminating candidates one at a time, until one has an overall majority.

However, some critics argue that the majority obtained by the winner of an IRV election is an artificial one. This is because there may be a candidate that voters prefer to the winner of an IRV election, but who has been eliminated because of garnering a small number of higher preferences. Advocates of this view argue that a candidate can only claim to have majority support if they are the 'Condorcet winner'–that is, the candidate voters prefer to every other candidate when compared to them one at a time. In fact, when IRV elects a candidate other than the Condorcet winner it will always be that the majority of voters prefer the Condorcet winner to the IRV winner (the only system that always elects the Condorcet winner is Condorcet's method). Defenders point out, however, that the Condorcet winner could be a candidate who did not win a single first choice ranking from voters and would have zero percent in a traditional plurality voting election. IRV seeks a balance between a candidate with strong support and a candidate with broad support.

IRV may be less likely to elect centrist candidates than some other preferential systems, such as Condorcet's method and the Borda count as long as candidates vote sincerely in those systems. For this reason it can be considered a less consensual system than these alternatives. Some IRV supporters consider this a strength, because an off-center candidate, with the enthusiastic support of many voters, may be preferable to a consensus candidate and that this candidate still must be accepted by a majority of voters.

IRV produces different results to Condorcet and the Borda count because it does not consider the lower preferences of all voters, only of those whose higher choices have been eliminated, and because of its system of sequential exclusions. IRV's process of excluding candidates one at a time can lead to the elimination, early in the count, of a candidate who, if they had remained in the count longer, would have received enough transfers to be elected.

[edit] Example

Consider again the preferences of the voters in the election in Example I above. This time the third preferences of the voters are important and so have been included:

# 39 voters 12 voters 7 voters 42 voters
1st Andrew Brian Brian Catherine
2nd Brian Andrew Catherine Brian
3rd Catherine Catherine Andrew Andrew

In a plurality election, Catherine would be elected. In a traditional runoff election, the voters would choose in a second round between Catherine and Andrew. In an IRV election Andrew will be elected. Under Condorcet's method or the Borda count Brian would win. Favouring Brian is the fact that a majority of voters prefer him to Andrew. This can be seen by the fact that 61 voters have given him a higher ranking than his opponent. Further, Andrew is ranked last by 49 voters, which seems to indicate that he is strongly disliked by almost half the electorate. Brian is either the first or second choice of every voter, which suggests that he is a broadly acceptable compromise candidate. On the other hand Andrew is the first preference of a large number of voters while Brian is the first choice of few. This might suggest that Andrew has the enthusiastic support of a large portion of the electorate, while Brian is a consensus candidate.

[edit] Alternative methods of eliminating candidates

Top-two IRV
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Top-two IRV

In the most common method of IRV, "if no candidate achieves an overall majority of first preferences, the candidate with the fewest first preferences is eliminated" (see Counting the Votes above). This is called bottom up elimination.

Some alternatives have been proposed that use other methods of eliminating candidates.

The Top-two IRV alternative simplifies the elimination process by duplicating the Two-round system of eliminating all but the top-two voted candidates in the first round. (London uses a version of this called Supplementary Vote with a further restriction that voters are only allowed to offer two rank choices.) Unlike the bottom-up elimination form of IRV, this variation does not allow for the possibility of a plurality-third or lower candidate to compete in the final round.

A practical benefit of the Top-two IRV process is expediency and confidence in the result with only two rounds. Most apparent in smaller elections, like with under 100 ballots among a dozen choices, confidence can be lost in a bottom-up elimination due to cumbersome ties on the bottom (or near ties affected by counting errors). Frequent and even multiple use of tie-breaking rules in one election will leave uncomfortable doubts over whether the winner might have changed if a recount was performed.

Another alternative known as the Coombs' method eliminates the candidate that ranks lowest on the most ballots. (All candidates must be ranked in order to apply this elimination rule.)

[edit] Evaluation by criteria

Scholars of electoral systems often compare them using mathematically-defined voting system criteria. IRV passes the majority criterion, the mutual majority criterion, the Condorcet loser criterion and, if the right tie-breaker method is used, the independence of clones criterion. IRV fails the monotonicity criterion, the consistency criterion, the Condorcet criterion, the participation criterion, reversal symmetry and the independence of irrelevant alternatives criterion.

[edit] See also

[edit] References

  1. ^ Australian Electoral Commission [1]

[edit] External links

Advocacy organisations
Opposition positions
Analysis
Examples
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