The Coombs' method, created by Clyde Coombs, is a voting system used for single-winner elections in which each voter rank-orders the candidates. It is very similar to Instant Runoff Voting (a US term; it is known as Preferential Voting in some other countries). Clyde Hamilton Coombs (July 22, 1912 - February 4, 1988) was an American psychologist specialized in the field of mathematical psychology. ... The Democracy Series Liberal democracy History of democracy Representative democracy Representation Voting Voting systems Elections Elections by country Elections by calender Politics Politics by country Political campaigns Political science Political philosophy Political parties Parties by country Parties by name Parties by ideology Voting systems are methods (algorithms) for groups of... The Elections and Parties Series Democracy Liberal democracy History of democracy Referenda Representative democracy Representation Voting Voting systems Elections Elections by country Elections by calender Electoral systems Politics Politics by country Political campaigns Political science Political philosophy Related topics Political parties Parties by country Parties by name Parties by ideology... The term preferential voting (or preference voting) has several different meanings: A ranked ballot or preferential voting system is a type of voting system in which each voter casts their vote by ranking candidates in order of preference. ... 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 until someone receives majority vote. ...

Procedures

Each voter rank-orders all of the candidates on their ballot. If at any time one candidate is ranked first (among non-eliminated candidates) by an absolute majority of the voters, then this is the winner. As long as this is not the case, the candidate which is ranked last (again among non-eliminated candidates) by the most (or a plurality of) voters is eliminated. (Conversely, in Instant Runoff Voting the candidate ranked first (among non-eliminated candidates) by the least amount of voters is eliminated.) A plurality (or relative majority) is the largest share of something, which may or may not be a majority. ... 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 until someone receives majority vote. ...

An example

Imagine an election to choose 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 miles northeast of Chattanooga). Here's the population breakdown by metro area (surrounding county): State nickname: Volunteer State Other U.S. States Capital Nashville Largest city Memphis (largest metropolitan area is Nashville) Governor Phil Bredesen (D) Official languages English Area 109,247 km² (36th)  - Land 106,846 km²  - Water 2,400 km² (2. ...

• 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 Tennessee follows from those population centers, one could easily envision an election where the percentages of sincere preferences would be as follows: Census. ...

Assuming all of the voters vote sincerely (strategic voting is discussed below), the results would be as follows, by percentage:

• In the first round, no candidate has an absolute majority of first place votes (51).
• Memphis, having the most last place votes (26+15+17=58), is therefore eliminated.
• In the second round, Memphis is out of the running, and so must be factored out. Memphis was ranked first on Group A's ballots, so the second choice of Group A, Nashville, gets an additional 42 first place votes, giving it an absolute majority of first place votes (68 versus 15+17=32) and making it thus the winner. Note that the last place votes are disregarded in the final round.

Note that although Coomb's method chose the Condorcet winner here, this is not necessarily the case. The Condorcet candidate or Condorcet winner of an election is the candidate who, when compared in turn with each of the other candidates, is preferred over the other candidate. ...

Potential for tactical voting

The Coombs' method is vulnerable to three strategies: compromising, push-over and teaming. In voting systems, tactical voting (or strategic voting) occurs when a voter misrepresents his or her sincere preferences in order to gain a more favorable outcome. ... Strategic nomination is the manipulation of an election through its candidate set (compare this to tactical voting, where the manipulation comes from the voters). ...

See also

• List of democracy and elections-related topics

Politics is the process and method of gaining or maintaining support for public or common action: the conduct of decision-making for groups. ...

External links

• Condorcet.org EMR: Coombs' method