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Revision as of 15:03, 14 July 2003 by DanKeshet (talk | contribs) (import example from Condorcet's method. rewrite tactical voting to use new example)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)Instant-runoff voting (known as alternative vote or the preferential system 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 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.
The Single Transferable Vote, and other multiple-winner methods, reduce to instant-runoff voting in the single-winner case.
The system is favored by many third parties, most notably the Green 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). This dilemma rose to attention in the United States in the 2000 election, when supporters of Ralph Nader found themselves caught in a dilemma between "voting their conscience" or opting for Al Gore in the interests of defeating George W. Bush.
Voting
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. Let's say the candidates for the capital are Memphis (on the far west end), Nashville (in the center), Chattanooga (129 miles southeast of Nashville), and Knoxville (on the far east side, 114 northeast of Chattanooga). Here's the population breakdown by metro area (surrounding county):
- Memphis (Shelby County): 826,330
- Nashville (Davidson County): 510,784
- Chattanooga (Hamilton County): 285,536
- Knoxville (Knox County): 335,749
Let's say that in the vote, the voters vote based on geographic proximity. Assuming that the population distribution of the rest of Tennesee follows from those population centers, one could easily envision an election where the percentages of votes would be as follows:
|
42% of voters (close to Memphis) |
26% of voters (close to Nashville) |
15% of voters (close to Chattanooga) |
17% of voters (close to Knoxville) |
The results would be tabulated as follows:
| City | Round 1 | Round 2 | Round 3 |
|---|---|---|---|
| Memphis | 42 | 42 | 42 |
| Nashville | 26 | 26 | |
| Chattanooga | 15 | 0 | |
| Knoxville | 17 |
In the first round, Chattanooga, having the smallest vote, is eliminated. All of the votes for Chattanooga have Knoxville have a second choice, so they are transferred to Knoxville. Now, Nashville has the smallest vote, so it is eliminated. The votes for Nashville have Chattanooga as a second choice, but as Chatanooga has been eliminated, they instead transfer to their third choice, Knoxville. Knoxville now has 58% of the vote, and it is the winner.
Potential for Tactical Voting
As instant-runoff voting fails the monotonicity criterion, there are situations in which a voter can get a better outcome for themselves through the tactical voting technique known as "compromise".
In the above example, 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.
Alternately, 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.
Impact on factions 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.
See also: Runoff voting, Single Transferable Vote, approval voting, Condorcet's method