This article does not cite any references or sources. (December 2007) Please help improve this article by adding citations to reliable sources. Unverifiable material may be challenged and removed.

In voting systems theory, the independence of clones criterion is a criterion that measures an election method's robustness to strategic nomination. The criterion, first formulated by Nicolaus Tideman^[1], states that the addition of a candidate identical to one already present in an election will not cause the winner of the election to change. Image File history File links Question_book-3. ... Strategic nomination is the manipulation of an election through its candidate set (compare this to tactical voting, where the manipulation comes from the voters). ... T. Nicolaus Tideman (born August 11, 1943 in Chicago, Illinois) is a Professor of Economics at Virginia Polytechnic Institute and State University. ...

In some systems, the introduction of a clone will tend to divide support between the similar candidates, making it less likely either will be elected. In some other systems, the presence of a clone will tend to reduce support for dissimilar candidates, making it more likely that one (or more) of the similar candidates will be elected. In yet other systems, the introduction of clones will not significantly affect the chances that one of the similar candidates will be elected. There are further systems where the effect of the introduction of clones depends on the distribution of other votes.

Clone Negative and Clone Positive

Elections methods that fail independence of clones can either be clone negative (the addition of an identical candidate will decrease a candidate’s chance of winning) or clone positive (the addition of an identical candidate will increase a candidate’s chance of winning).

The Borda count is an example of a clone positive method. Plurality is an example of a clone negative method. The Borda count is a single winner election method in which voters rank candidates in order of preference. ... An example of a plurality ballot. ...

An election method that fails the independence of clones criterion in the direction of being clone negative is also sometimes said to fail Clone-Winner, whereas a method that fails it in the direction of being clone positive fails Clone-Loser; the terms were originally used by Douglas Woodall.

Complying Methods

Instant-runoff voting, approval voting and range voting meet the independence of clones criterion. Many election methods that comply with the Condorcet criterion such as Ranked pairs and Schulze^[2] also meet independence of clones. Example Instant-runoff voting ballot Instant-runoff voting (IRV) is a voting system most commonly used for single member elections in which voters have one vote, but can rank candidates in order of preference. ... On an approval ballot, the voter can vote for any number of candidates. ... Range voting (also called ratings summation, average voting, cardinal ratings, 0–99 voting, or the score system or point system) is a voting system for one-seat elections under which voters score each candidate, the scores are added up, and the candidate with the highest score wins. ... 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. ... It has been suggested that Maximize Affirmed Majorities be merged into this article or section. ... The Schulze method is a voting system developed in 1997 by Markus Schulze that selects a single winner using votes that express preferences. ...

The Borda count, two-round system, Bucklin voting and plurality fail the independence of clones criterion. The Borda count is a single winner election method in which voters rank candidates in order of preference. ... An example of runoff voting. ... Bucklin is a voting system that can be used for single-member districts and also multi-member districts. ...

Examples of failure of the independence of clones criterion

Plurality

For example in an election there are two candidates A and B. Candidate A receives 100 votes and candidate B receives 75 votes. A clone of candidate A, called "candidate A(clone)" is added to the candidate set. The votes are now A 50, A(clone) 50 and B 75.

As a result of the presence on the ballot of another candidate identical to candidate A the winner of the election has changed from A to B.

Borda Count

For example in an election there are two candidates A and B. The following votes are cast:

A obtains 55 Borda points (55 x 1 + 45 x 0) and B obtains 45 Borda points (55 x 0 + 45 x 1).

Candidate A is the winner.

A candidate identical to candidate B candidate B(clone) is added to the candidate set. The following votes are now cast:

Candidate A has 110 Borda points (55 x 2 + 45 x 0). Candidate B has 145 Borda points (55 x 1 + 45 x 2). Candidate B(clone) has 45 Borda points (55 x 0 + 45 x 1).

As a result of the addition of candidate B(clone) to the candidate set the winner of the election has changed from A to B.

References

See also

Strategic nomination Strategic nomination is the manipulation of an election through its candidate set (compare this to tactical voting, where the manipulation comes from the voters). ...