Kemeny-Young method
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The Kemeny-Young method is a voting system that uses preferential ballots, pairwise comparison counts, and sequence scores to identify the most popular choice, and also identify the second-most popular choice, the third-most popular choice, and so on down to the least-popular choice. This is a Condorcet method because if there is a Condorcet winner, it will always be ranked as the most popular choice.

The Kemeny-Young method is also known as the maximum likelihood method, the linear ordering problem, VoteFair popularity ranking, and the median relation.

Description

The Kemeny-Young method uses preferential ballots on which a voter ranks the choices according to their order of preference. A voter is allowed to rank more than one choice at the same preference level. Unranked choices are usually interpreted as least-preferred.

Kemeny-Young calculations are usually done in two steps. The first step is to create a matrix or table that counts pairwise voter preferences. The second step is to test all possible order-of-preference sequences, calculate a sequence score for each sequence, and compare the scores. Each sequence score equals the sum of the pairwise counts that apply to the sequence, and the sequence with the highest score is identified as the overall ranking, from most popular to least popular.

Example

Imagine that Tennessee is having an election on the location of its capital. The population of Tennessee is concentrated around its four major cities, which are spread throughout the state. For this example, suppose that the entire electorate lives in these four cities, and that everyone wants to live as near the capital as possible.

The candidates for the capital are:

• Memphis, the state's largest city, with 42% of the voters, but located far from the other cities
• Nashville, with 26% of the voters
• Knoxville, with 17% of the voters
• Chattanooga, with 15% of the voters

The preferences of the voters would be divided like this:

This matrix summarizes the corresponding pairwise comparison counts:

The Kemeny-Young method arranges the pairwise comparison counts in the following tally table:

The Kemeny sequence score for the sequence Memphis first, Nashville second, Chattanooga third, and Knoxville fourth equals (the unit-less number) 345, which is the sum of the following annotated numbers.

42% (of the voters) prefer Memphis over Nashville

42% prefer Memphis over Chattanooga

42% prefer Memphis over Knoxville

68% prefer Nashville over Chattanooga

68% prefer Nashville over Knoxville

83% prefer Chattanooga over Knoxville

This table lists all the sequence scores.

The highest sequence score is 393, and this score is associated with the following sequence, so this is the winning preference order.

If a single winner is needed, the first choice, Nashville, is chosen. (In this example Nashville is the Condorcet winner.)

Characteristics

In all cases that do not result in an exact tie, the Kemeny-Young method identifies a most-popular choice, second-most popular choice, and so on.

A tie can occur at any preference level. Except in some cases where circular ambiguities are involved, the Kemeny-Young method only produces a tie at a preference level when the number of voters with one preference exactly matches the number of voters with the opposite preference.

Satisfied criteria for all Condorcet methods

All Condorcet methods, including the Kemeny-Young method, satisfy these criteria:

Non-imposition

There are voter preferences that can yield every possible overall order-of-preference result, including ties at any combination of preference levels.

Condorcet criterion

If there is a choice that wins all pairwise contests, then this choice wins.

Majority criterion

If a majority of voters strictly prefer choice X to every other choice, then choice X is identified as the most popular.

Pareto efficiency

Any pairwise preference expressed by every voter results in the preferred choice being ranked higher than the less-preferred choice.

Non-dictatorship

A single voter cannot control the outcome in all cases.

Additional satisfied criteria

The Kemeny-Young method also satisfies these criteria:

Universality

Identifies the overall order of preference for all the choices. The method does this for all possible sets of voter preferences and always produces the same result for the same set of voter preferences.

Monotonicity

If voters increase a choice's preference level, the ranking result either does not change or the promoted choice increases in overall popularity.

Smith criterion

The most popular choice is a member of the Smith set, which is the smallest set of choices such that every member of the set is pairwise preferred to every choice not in the Smith set.

Local independence of irrelevant alternatives

If choice X is not in the Smith set, adding or withdrawing choice X does not change a result in which choice Y is identified as most popular.

Reinforcement

If all the ballots are divided into separate races and the overall ranking for the separate races are the same, then the same ranking occurs when all the ballots are combined.

Reversal symmetry

If the preferences on every ballot are inverted, then the previously most popular choice must not remain the most popular choice.

Failed criteria for all Condorcet methods

In common with all Condorcet methods, the Kemeny-Young method fails these criteria (which means the described criteria do not apply to the Kemeny-Young method):

Independence of irrelevant alternatives

Adding or withdrawing choice X does not change a result in which choice Y is identified as most popular.

Invulnerability to burying

A voter cannot displace a choice from most popular by giving the choice an insincerely low ranking.

Invulnerability to compromising

A voter cannot cause a choice to become the most popular by giving the choice an insincerely high ranking.

Participation

Adding ballots that rank choice X over choice Y never cause choice Y, instead of choice X, to become most popular.

Later-no-harm

Ranking an additional choice (that was otherwise unranked) cannot displace a choice from being identified as the most popular.

Consistency

If all the ballots are divided into separate races and choice X is identified as the most popular in every such race, then choice X is the most popular when all the ballots are combined.

Additional failed criteria

The Kemeny-Young method also fails these criteria (which means the described criteria do not apply to the Kemeny-Young method):

Independence of clones

Offering a larger number of similar choices, instead of offering only a single such choice, does not change the probability that one of these choices is identified as most popular.

Invulnerability to push-over

A voter cannot cause choice X to become the most popular by giving choice Y an insincerely high ranking.

Schwartz

The choice identified as most popular is a member of the Schwartz set.

Polynomial runtime^[1]

An algorithm is known to determine the winner using this method in a runtime that is polynomial in the number of choices.

Calculation methods

Calculating all sequence scores requires time proportional to N!, where N is the number of choices. Although one need not compute scores to find the winner, any algorithm finding the winner requires superpolynomial time (unless P=NP). Nevertheless, fast calculation methods based on linear programming allow the computation of full rankings for votes with as many as 40 choices.^[2]

History

The Kemeny-Young method was developed by John Kemeny in 1959.^[3]

In 1978 Peyton Young and Arthur Levenglick showed^[4] that this method was the unique neutral method satisfying reinforcement and the Condorcet criterion. In another paper^[5], Young argued that the Kemeny-Young method was a possible interpretation of one of Condorcet's proposals.

Since 1991 the method has been promoted under the name "VoteFair popularity ranking" by Richard Fobes.^[6]

References

1. ^ J. Bartholdi III, C. A. Tovey, and M. A. Trick, "Voting schemes for which it can be difficult to tell who won the election", Social Choice and Welfare, Vol. 6, No. 2 (1989), pp. 157–165.
2. ^ Vincent Conitzer, Andrew Davenport, and Jayant Kalagnanam, "Improved bounds for computing Kemeny rankings" (2006).
3. ^ John Kemeny, "Mathematics without numbers", Daedalus 88 (1959), pp. 577–591.
4. ^ H. P. Young and A. Levenglick, "A Consistent Extension of Condorcet's Election Principle", SIAM Journal on Applied Mathematics 35, no. 2 (1978), pp. 285–300.
5. ^ H. P. Young, "Condorcet's Theory of Voting", American Political Science Review 82, no. 2 (1988), pp. 1231-1244.
6. ^ Richard Fobes, "The Creative Problem Solver's Toolbox", (ISBN 0-9632-2210-4), 1993, pp. 223-225.

External links

• VoteFair.org — A website that calculates Kemeny-Young results. For comparison, it also calculates the winner according to plurality, Condorcet, Borda count, and other voting methods.