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Instant-runoff voting or one-ballot runoff (known as alternative vote in many countries, and as preference voting or single-transfer voting by academics which describe specific systems) is one of a set of related voting systems used for elections in single-member districts, or selecting leaders within organizations (including political parties, which usually employ runoff voting proper).

The simplest form of instant-runoff voting is approval voting, where the runoff is trivial, since preferred and compromise canadidates or measures are all tallied at once, in one round, count equally, and must be accepted or not with no statement of preference. There are no numbers on the ballot, merely checkmarks, or nothing.

All other forms of instant-runoff involve a more involved system of preference voting. It is this feature, rather than the instant tally, that is remarked by most commentators and advocates of electoral reform. However simplicity of ballot, and other more recent arguments that preference voting may lead to more compromise candidates even than approval voting, argue for approval. This article will refer to preference voting specifically as this is by far the most common system in use today, and to most advocates it is the default system of instant-runoff voting.

Preference voting is used, among other places, to elect the House of Representatives in Australia and the president of the Republic of Ireland. It is rarely used in the United States, but in March 2002 it was adopted by voters as the means of electing local candidates in San Francisco. Suggested by Robert's Rules of Order, it is increasingly used in the United States for non-governmental elections, including student elections at Harvard, MIT, Stanford, the Universities of Illinois and Maryland, Vassar and William and Mary.

This system 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.

Voting

Each voter ranks at least one candidate in order of preference. In most Australian elections, voters are required to rank all candidates - one of several variations that tend to have major impacts on the strategies of parties.

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 is repeated until one candidate is the most favored choice of more than fifty percent of voters.

An example

Four candidates: Andrea, Brad, Carter, and Delilah.

12 voters rank the candidates:

1. Andrea
2. Brad
3. Carter
4. Delilah

8 voters rank the candidates:

1. Carter
2. Brad
3. Delilah
4. Andrea

4 voters rank the candidates:

1. Delilah
2. Brad
3. Carter
4. Andrea

1 voter ranked the candidates:

1. Brad
2. Carter
3. Andrea
4. Delilah

As none of the candidates have reached 50%, the lowest-ranked candidate, Brad, is eliminated. The one voter for him now has his or her ballot count for Carter. The vote table now stands:

• Andrea: 12
• Carter: 9
• Delilah: 4

Delilah is eliminated. The four voters who supported her have their ballots count for the next eligible candidate on their ballots, which turns out to be Carter.

The vote table now stands:

• Carter: 13
• Andrea: 12

Carter is elected.

Potential for Tactical Voting

Tactical voting is more difficult under preference voting than under plurality voting or standard runoff voting, but easier than under approval voting. However, it is not impossible. The basic premise of tactical voting within preference voting is to ensure that the proper mix of candidates are left standing toward the end.

For example, suppose there are three candidates: Andrea, Brad, and Carter. It is expected (maybe due to polling) that Andrea will receive 40% of the initial vote, Brad 31%, and Carter 29%. It is also expected that all of Carter's support will prefer Brad to Andrea, whereas half of Brad's support prefer Andrea to Carter. This is not an absurd situation if you say that Andrea is left-of-center, Brad centrist, and Carter right-of-center. In order to attain victory in the final round, some of Andrea's supporters may break off and instead vote Carter first, then Andrea. This would lift Carter to victory over Brad in the first round, after which, Brad's votes, evenly split, lift Andrea to victory in the second round. This scenario is identical to one that may occur in standard runoff voting.

See also: approval voting, Runoff voting, Single-transfer voting

External links

• Center for Voting and Democracy
• instant runoff