Ranked pairs

Ranked pairs (or RP), sometimes called the Tideman method, is a tournament-style system of ranked-choice voting first proposed by Nicolaus Tideman in 1987. ^{[1] [2]}

Ranked pairs begins with a round-robin tournament, where the one-on-one margins of victory for each candidate are compared to find a majority winner. If there is a Condorcet cycle (a rock-paper-scissors sequence A > B > C > A), the cycle is broken by dropping nearly-tied elections, i.e. the closest elections in the cycle. ^[3]

Procedure

The ranked pairs procedure is as follows:

1. Consider each pair of candidates round-robin style and calculate the pairwise margin for each in a one-on-one matchup.
2. Sort the pairs by the (absolute) margin of victory, going from largest to smallest.
3. Going down the list, check whether adding each matchup would create a cycle. If it would, cross out the election; this will be the election(s) in the cycle with the smallest margin of victory (near-ties). ^{[note 1]}

At the end of this procedure, all cycles will be eliminated, leaving a unique winner who wins every one-on-one matchup (that has not been crossed out). The lack of cycles means that candidates can be ranked linearly based on the matchups that have been left behind.

The River variant crosses out redundant (duplicate) defeats as well as cyclical ones. In other words, it does not "double-count" multiple defeats for the same candidate. The River method allows for faster computation of winners. ^{[note 2]} It also makes the election method less vulnerable to strategic nominations, by preventing weak candidates from creating several cycles to manipulate the outcome.

Example

The situation

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Suppose that Tennessee is holding an election on the location of its capital. The population is concentrated around four major cities. All voters want the capital to be as close to them as possible. The options are:

• Memphis, the largest city, but far from the others (42% of voters)
• Nashville, near the center of the state (26% of voters)
• Chattanooga, somewhat east (15% of voters)
• Knoxville, far to the northeast (17% of voters)

The preferences of each region's voters are:

42% of voters Far-West	26% of voters Center	15% of voters Center-East	17% of voters Far-East
Memphis Nashville Chattanooga Knoxville	Nashville Chattanooga Knoxville Memphis	Chattanooga Knoxville Nashville Memphis	Knoxville Chattanooga Nashville Memphis

The results are tabulated as follows:

Pairwise election results

A B	Memphis	Nashville	Chattanooga	Knoxville
Memphis		[A] 58% [B] 42%	[A] 58% [B] 42%	[A] 58% [B] 42%
Nashville	[A] 42% [B] 58%		[A] 32% [B] 68%	[A] 32% [B] 68%
Chattanooga	[A] 42% [B] 58%	[A] 68% [B] 32%		[A] 17% [B] 83%
Knoxville	[A] 42% [B] 58%	[A] 68% [B] 32%	[A] 83% [B] 17%

• [A] indicates voters who preferred the candidate listed in the column caption to the candidate listed in the row caption
• [B] indicates voters who preferred the candidate listed in the row caption to the candidate listed in the column caption

Tally

First, list every pair, and determine the winner:

Pair	Winner
Memphis (42%) vs. Nashville (58%)	Nashville 58%
Memphis (42%) vs. Chattanooga (58%)	Chattanooga 58%
Memphis (42%) vs. Knoxville (58%)	Knoxville 58%
Nashville (68%) vs. Chattanooga (32%)	Nashville 68%
Nashville (68%) vs. Knoxville (32%)	Nashville 68%
Chattanooga (83%) vs. Knoxville (17%)	Chattanooga: 83%

The votes are then sorted. The largest majority is "Chattanooga over Knoxville"; 83% of the voters prefer Chattanooga. Thus, the pairs from above would be sorted this way:

Pair	Winner
Chattanooga (83%) vs. Knoxville (17%)	Chattanooga 83%
Nashville (68%) vs. Knoxville (32%)	Nashville 68%
Nashville (68%) vs. Chattanooga (32%)	Nashville 68%
Memphis (42%) vs. Nashville (58%)	Nashville 58%
Memphis (42%) vs. Chattanooga (58%)	Chattanooga 58%
Memphis (42%) vs. Knoxville (58%)	Knoxville 58%

Lock

The pairs are then locked in order, skipping any pairs that would create a cycle:

• Lock Chattanooga over Knoxville.
• Lock Nashville over Knoxville.
• Lock Nashville over Chattanooga.
• Lock Nashville over Memphis.
• Lock Chattanooga over Memphis.
• Lock Knoxville over Memphis.

In this case, no cycles are created by any of the pairs, so every single one is locked in.

Every "lock in" would add another arrow to the graph showing the relationship between the candidates. Here is the final graph (where arrows point away from the winner).

In this example, Nashville is the winner using the ranked-pairs procedure. Nashville is followed by Chattanooga, Knoxville, and Memphis in second, third, and fourth places respectively.

Summary

In the example election, the winner is Nashville. This would be true for any Condorcet method.

Under first-past-the-post and some other systems, Memphis would have won the election by having the most people, even though Nashville won every simulated pairwise election outright. Using instant-runoff voting in this example would result in Knoxville winning even though more people preferred Nashville over Knoxville.

Criteria

Of the formal voting criteria, the ranked pairs method passes the majority criterion, the monotonicity criterion, the Smith criterion (which implies the Condorcet criterion), the Condorcet loser criterion, and the independence of clones criterion. Ranked pairs fails the consistency criterion and the participation criterion. While ranked pairs is not fully independent of irrelevant alternatives, it still satisfies local independence of irrelevant alternatives ^{[ broken anchor ]} and independence of Smith-dominated alternatives, meaning it is likely to roughly satisfy IIA "in practice."

Independence of irrelevant alternatives

Ranked pairs fails independence of irrelevant alternatives, like all other ranked voting systems. However, the method adheres to a less strict property, sometimes called independence of Smith-dominated alternatives (ISDA). It says that if one candidate (X) wins an election, and a new alternative (Y) is added, X will win the election if Y is not in the Smith set. ISDA implies the Condorcet criterion.

The River variant satisfies an additional criterion, independence of Pareto-dominated alternatives, which says that if any candidate would lose some election unanimously, they must not affect the outcome.

Comparison table

The following table compares ranked pairs with other preferential single-winner election methods:

Comparison of voting systems

Criterion:	Majority	Majority loser criterion	Mutual majority criterion	Condorcet winner	Condorcet loser	Smith	ISDA	LIIA	Cloneproof	Monotone	Participation	Reversal	Later-no-harm	Later-no-help	Polynomial time	Resolvability
Schulze	Yes	Yes	Yes	Yes	Yes	Yes	Yes	No	Yes	Yes	No	Yes	No	No	Yes	Yes
Ranked pairs	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	No	Yes	No	No	Yes	Yes
Tideman alternative	Yes	Yes	Yes	Yes	Yes	Yes	Yes	No	Yes	No	No	No	No	No	Yes	Yes
Kemeny–Young	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	No	Yes	No	Yes	No	No	No	Yes
Copeland	Yes	Yes	Yes	Yes	Yes	Yes	Yes	No	No	Yes	No	Yes	No	No	Yes	No
Nanson	Yes	Yes	Yes	Yes	Yes	Yes	No	No	No	No	No	Yes	No	No	Yes	Yes
Black	Yes	Yes	No	Yes	Yes	No	No	No	No	Yes	No	Yes	No	No	Yes	Yes
Instant-runoff voting	Yes	Yes	Yes	No	Yes	No	No	No	Yes	No	No	No	Yes	Yes	Yes	Yes
Borda count	No	Yes	No	No	Yes	No	No	No	No	Yes	Yes	Yes	No	Yes	Yes	Yes
Baldwin	Yes	Yes	Yes	Yes	Yes	Yes	No	No	No	No	No	No	No	No	Yes	Yes
Bucklin	Yes	Yes	Yes	No	No	No	No	No	No	Yes	No	No	No	Yes	Yes	Yes
Plurality	Yes	No	No	No	No	No	No	No	No	Yes	Yes	No	Yes	Yes	Yes	Yes
Coombs	Yes	Yes	Yes	No	Yes	No	No	No	No	No	No	No	No	No	Yes	Yes
Minimax	Yes	No	No	Yes	No	No	No	No	No	Yes	No	No	No	No	Yes	Yes
Anti-plurality	No	Yes	No	No	No	No	No	No	No	Yes	Yes	No	No	No	Yes	Yes
Dodgson	Yes	No	No	Yes	No	No	No	No	No	No	No	No	No	No	No	Yes

Notes

1. Rather than crossing out near-ties, step 3 is sometimes described as going down the list and confirming ("locking in") the largest victories that do not create a cycle, then ignoring any victories that are not locked-in.
2. A full ranking with River is no faster than a full ranking with RP. It can be constructed by finding the first-place winner, then rerunning the election without the first-place winner to get a second-place winner, etc.

References

1. Tideman, T. N. (1987-09-01). "Independence of clones as a criterion for voting rules". Social Choice and Welfare. 4 (3): 185–206. doi:10.1007/BF00433944. ISSN   1432-217X. S2CID   122758840.
2. Schulze, Markus (October 2003). "A New Monotonic and Clone-Independent Single-Winner Election Method". Voting matters (www.votingmatters.org.uk). 17. McDougall Trust. Archived from the original on 2020-07-11. Retrieved 2021-02-02.
3. Munger, Charles T. (2022). "The best Condorcet-compatible election method: Ranked Pairs". Constitutional Political Economy . 34 (3): 434–444. doi: 10.1007/s10602-022-09382-w .

External links

• Descriptions of ranked-ballot voting methods by Rob LeGrand
• Example JS implementation by Asaf Haddad
• Pair Ranking Ruby Gem by Bala Paranj
• A margin-based PHP Implementation of Tideman's Ranked Pairs
• Rust implementation of Ranked Pairs by Cory Dickson