Instant Runoff Voting

Instant Runoff Voting (known as Alternative Vote in many countries) is a Voting system used for elections in single-member districts. It is used, among other places, to elect the House of Representatives in Australia.

This system encourages popularity over acceptability: any bloc of more than half the voters can elect a candidate regardless of the opinion of the rest of the voters. Candidates must also receive enough initial first-place votes to stay involved in the election, even if they have a large number of second- or third- place votes.

Voting

Each voter ranks all candidates in order of preference.

Counting The Votes

Top-preference votes are tallied. The candidate with the least support is eliminated, and their votes are reallocated to the next-highest ranked candidates on the eliminated ballots. After a candidate is eliminated, they may not receive any more votes.

This process is repeated until one candidate has received fifty percent of the votes cast.

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 candidats:

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 vote for him is transferred to Carter. The vote table now stands:

Andrea: 13 Carter: 9 Delilah: 4

Delilah is eliminated. The four votes she received are transferred to the next eligible candidate on the ballots that voted for her, Carter. (Brad has been eliminated.) Carter has now received 13 votes and he is elected.

Potential for Tactical Voting

Tactical voting is more difficult under IRV than under plurality voting or standard runoff voting. However, it is not impossible. The basic premise of tactical voting in IRV 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: Runoff voting, Single transfer voting