Ranked voting

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Various types of ranked voting ballot

Ranked voting is any voting system that uses voters' orderings (rankings) of candidates to choose a single winner or multiple winners. More formally, a ranked rule is one that depends only on which of two candidates is preferred by a voter, and as such does not incorporate any information about intensity of preferences. Ranked voting systems vary dramatically in how preferences are tabulated and counted, which gives them very different properties.

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For example, in the Borda method, the 1st, 2nd, 3rd... candidates on each ballot receive 1, 2, 3… points, and the candidate with the smallest number of points is elected. In instant-runoff (ranked-choice) voting, lower preferences are used as contingencies (back-up preferences), and are only applied when all higher-ranked preferences on a ballot have been eliminated.

Ranked voting systems are usually contrasted with rated voting methods, which allow voters to indicate how strongly they support different candidates (e.g. on a scale from 0-10). [1] Ranked vote systems (ordinal systems) produce more information than X voting systems such as first-past-the-post voting. Rated voting systems use more information than ordinal ballots; as a result, they are not subject to many of the problems with ranked voting (including results like Arrow's theorem). [2] [3] [4]

Although not typically described as such, the most common ranked voting system is the well-known plurality rule, where each voter gives a single point to the candidate ranked first and zero points to all others. The most common non-degenerate ranked voting rule is the closely-related instant-runoff (ranked-choice) voting, a staged variant of the plurality system that repeatedly eliminates last-place plurality winners. [5]

In the United States and Australia, the terms ranked-choice voting and preferential voting are usually used to refer to the alternative or single transferable vote, a misnomer arising by way of conflation. However, these terms have also been used to mean ranked voting systems in general, leading most social choice theorists to recommend the use of more precise terms like instant-runoff voting (IRV).

History of ranked voting

The earliest known proposals for a ranked voting system other than plurality can be traced to the works of Ramon Llull in the late 13th century, who developed what would later be known as Copeland's method. Copeland's method was devised by Ramon Llull in his 1299 treatise Ars Electionis, which was discussed by Nicholas of Cusa in the fifteenth century. [6] [7]

A second wave of analysis began when Jean-Charles de Borda published a paper in 1781, advocating for the Borda count, which he called the "order of merit". This methodology drew criticism from the Marquis de Condorcet, who developed his own methods after arguing Borda's approach did not accurately reflect group preferences, because it was vulnerable to spoiler effects and did not always elect the majority-preferred candidate. [6]

Interest in ranked voting continued throughout the 19th century. Danish pioneer Carl Andræ formulated the single transferable vote (STV), which was adopted by his native Denmark in 1855. Condorcet had previously considered the single-winner version of it, the instant-runoff system, but immediately rejected it as pathological. [8] [9]

Theoretical exploration of electoral processes was revived by a 1948 paper from Duncan Black [10] and Kenneth Arrow's investigations into social choice theory, a branch of welfare economics that extends rational choice to include community decision-making processes. [11]

Adoption

Plurality voting is the most common voting system, and has been in widespread use since the earliest democracies.[ citation needed ]

The single transferable vote (STV) system was first invented by Carl Andræ in Denmark, where it was used briefly before being abandoned.[ citation needed ] It was later rediscovered by British lawyer Thomas Hare, whose writings soon spread the method throughout the British Empire. Tasmania adopted the method in the 1890s, with broader adoption throughout Australia beginning in the 1910s and 1920s. [12] It has been adopted in Ireland, South Africa, Malta, and approximately 20 cities each in the United States and Canada.[ citation needed ]

In more recent years, STV has seen a comeback in the United States. In November 2016, the voters of Maine narrowly passed Question 5, approving ranked-choice voting for all elections. This was first put to use in 2018, marking the inaugural use of a ranked choice voting system in a statewide election in the United States. Later, in November 2020, Alaska voters passed Measure 2, bringing ranked choice voting into effect from 2022. [13] [14] However, as before, the system has faced strong opposition. After a series of electoral pathologies in Alaska's 2022 congressional special election, a poll found 54% of Alaskans supported a repeal of the system; this included a third of the voters who had supported Peltola, the ultimate winner in the election. [15]

In the United States, single-winner ranked voting is used to elect politicians in Maine [16] and Alaska. [17] Nauru uses a positional method called the Dowdall system. Some local elections in New Zealand use the single transferable vote. [18]

Equal-ranked ballots

In voting with ranked ballots, a tied or equal-rank ballot is one where multiple candidates receive the same rank or rating.

In ranked-choice runoff and first-preference plurality, such ballots are generally discarded in practice. However, in social choice theory these methods are generally modeled by assuming equal-ranked ballots are "split" evenly between all equal-ranked candidates (e.g. in a two-way tie, each candidate receives half a vote).

By contrast, the Borda count and Condorcet methods can use different rules for handling equal-ranks. Such rules turn out to have extremely different mathematical properties and behaviors, particularly under strategic voting.

Theoretical foundations of ranked voting

Majority-rule

Many concepts formulated by the Marquis de Condorcet in the 18th century continue to significantly impact the field. One of these concepts is the Condorcet winner, the candidate preferred over all others by a majority of voters. A voting system that always elects this candidate is called a Condorcet method.

However, it is possible for an election to have no Condorcet winner, a situation called a Condorcet cycle. Suppose an election with 3 candidates A, B, and C has 3 voters. One votes A–C–B, one votes B–A–C, and one votes C–B–A. In this case, no Condorcet winner exists: A cannot be a Condorcet winner as two-thirds of voters prefer B over A. Similarly, B cannot be the winner as two-thirds prefer C over B, and C cannot win as two-thirds prefer A over C. This forms a rock-paper-scissors style cycle with no Condorcet winner.

Social well-being

Voting systems can also be judged on their ability to deliver results that maximize the overall well-being of society, i.e. to choose the best candidate for society as a whole. [19]

Spatial voting models

Spatial voting models, initially proposed by Duncan Black and further developed by Anthony Downs, provide a theoretical framework for understanding electoral behavior. In these models, each voter and candidate is positioned within an ideological space that can span multiple dimensions. It is assumed that voters tend to favor candidates who closely align with their ideological position over those more distant. A political spectrum is an example of a one-dimensional spatial model.

A spatial model of voting IRVCopeland.png
A spatial model of voting

The accompanying diagram presents a simple one-dimensional spatial model, illustrating the voting methods discussed in subsequent sections of this article. It is assumed that supporters of candidate A cast their votes in the order of A-B-C, while candidate C's supporters vote in the sequence of C-B-A. Supporters of candidate B are equally divided between listing A or C as their second preference. From the data in the accompanying table, if there are 100 voters, the distribution of ballots will reflect the positioning of voters and candidates along the ideological spectrum.

Spatial models offer significant insights because they provide an intuitive visualization of voter preferences. These models give rise to an influential theorem—the median voter theorem—attributed to Duncan Black. This theorem stipulates that within a broad range of spatial models, including all one-dimensional models and all symmetric models across multiple dimensions, a Condorcet winner is guaranteed to exist. Moreover, this winner is the candidate closest to the median of the voter distribution.

Empirical research has generally found that spatial voting models give a highly accurate explanation of most voting behavior. [20]

Other theorems

Arrow's impossibility theorem is a generalization of Condorcet's result on the impossibility of majority rule. It demonstrates that every ranked voting algorithm is susceptible to the spoiler effect. Gibbard's theorem provides a closely-related corollary, that no voting rule can have a single, always-best strategy that does not depend on other voters' ballots.

Examples

Borda count

The Borda count is a ranking system that assigns scores to each candidate based on their position in each ballot. If m is the total number of candidates, the candidate ranked first on a ballot receives m - 1 points, the second receives m - 2, and so on, until the last-ranked candidate who receives zero. In the given example, candidate B emerges as the winner with 130 out of a total 300 points. While the Borda count is simple to administer, it does not meet the Condorcet criterion. It is heavily affected by the entry of candidates who have no real chance of winning.

Other positional systems

Systems that award points in a similar way but possibly with a different formula are called positional systems. The score vector (m - 1, m - 2,..., 0) is associated with the Borda count, (1, 1/2, 1/3,..., 1/m) defines the Dowdall system and (1, 0,... 0) equates to first-past-the-post.

Instant-runoff (Ranked-choice) voting

Instant-runoff voting, often conflated with ranked-choice voting in general, is a voting method that recursively eliminates the plurality loser of an election until only one candidate is left.

In the given example, Candidate A is declared winner in the third round, having received a majority of votes through the accumulation of first-choice votes and redistributed votes from Candidate B. This system embodies the voters' preferences between the final candidates, stopping when a candidate garners the preference of a majority of voters.

IRV is notable in that it does not fulfill the Condorcet winner criterion, and as a result will not always elect majority-preferred candidate.

Defeat-dropping Condorcet

The defeat-dropping Condorcet methods all look for a Condorcet winner, i.e. a candidate who is not defeated by any other candidate in a one-on-one majority vote. If there is no Condorcet winner, they repeatedly drop (set the margin to zero) for the one-on-one matchups that are closest to being tied, until there is a Condorcet winner. How "closest to being tied" is defined depends on the specific rule. For minimax, the elections with the smallest margin of victory are dropped, whereas in ranked pairs only elections that create a cycle are eligible to be dropped (with defeats being dropped based on the margin of victory).

See also

Related Research Articles

<span class="mw-page-title-main">Two-round system</span> Voting system

The two-round system, also called ballotage, top-two runoff, or two-round plurality, is a single winner voting method. It is sometimes called plurality-runoff, although this term can also be used for other, closely-related systems such as ranked-choice voting or the exhaustive ballot. In a two-round system, both rounds are held under choose-one voting, where the voter marks a single favorite candidate. The two candidates with the most votes in the first round proceed to a second round, where all other candidates are excluded.

In social choice theory, Condorcet's voting paradox is a fundamental discovery by the Marquis de Condorcet that majority rule is inherently self-contradictory. The result states that it is logically impossible for any voting system to guarantee the winner has a majority of the vote, because it is possible no such winner exists: in some situations, a majority of voters will prefer A to B, B to C, and also C to A, even if every voter's individual preferences are rational and avoid self-contradiction. Examples of Condorcet's paradox are called Condorcet cycles or cyclic ties.

In social choice theory and politics, the spoiler effect or Arrow's paradox refers to a situation where a losing spoiler candidate affects the results of an election. A voting system that is not affected by spoilers satisfies independence of irrelevant alternatives or independence of spoilers.

<span class="mw-page-title-main">Condorcet method</span> Pairwise-comparison electoral system

A Condorcet method is an election method that elects the candidate who wins a majority of the vote in every head-to-head election against each of the other candidates, whenever there is such a candidate. A candidate with this property, the pairwise champion or beats-all winner, is formally called the Condorcet winner. The head-to-head elections need not be done separately; a voter's choice within any given pair can be determined from the ranking.

Arrow's impossibility theorem is a key result in social choice, discovered by Kenneth Arrow, showing that no ranked voting rule can behave rationally. Specifically, any such rule violates independence of irrelevant alternatives, the idea that a choice between and should not depend on the quality of a third, unrelated option . The result is most often cited in election science and voting theory, where is called a spoiler candidate. In this context, Arrow's theorem can be restated as showing that no ranked voting rule can eliminate the spoiler effect.

The Copeland or Llull method is a ranked-choice voting system based on counting each candidate's pairwise wins and losses.

<span class="mw-page-title-main">Monotonicity criterion</span> Property of electoral systems

The positive response/association, monotonicity, or nonperversitycriterion is a principle of social choice theory that says that increasing a candidate's ranking or rating should not cause them to lose. Positive response rules out cases where a candidate loses an election as a result of receiving too much support from voters. Rules that violate positive response are also called perverse and are said to suffer the more-is-less paradox. Such paradoxes are especially common in ranked-choice voting (RCV-IRV), a behavior which can lead to the elimination of moderate candidates and the election of extremists.

Independence of irrelevant alternatives (IIA) is a major axiom of decision theory which codifies the intuition that a choice between and should not depend on the quality of a third, unrelated outcome . There are several different variations of this axiom, which are generally equivalent under mild conditions. As a result of its importance, the axiom has been independently rediscovered in various forms across a wide variety of fields, including economics, cognitive science, social choice, fair division, rational choice, artificial intelligence, probability, and game theory. It is closely tied to many of the most important theorems in these fields, including Arrow's impossibility theorem, the Balinski-Young theorem, and the money pump arguments.

In an election, a candidate is called a majority winner or majority-preferred candidate if more than half of all voters would support them in a one-on-one race against any one of their opponents. Voting systems where a majority winner will always win are said to satisfy the majority-rule principle, because they extend the principle of majority rule to elections with multiple candidates.

In political science and social choice theory, Black'smedian voter theorem states that if voters and candidates are distributed along a one-dimensional spectrum and voters have single peaked preferences, any voting method that is compatible with majority-rule will elect the candidate preferred by the median voter. The median voter theorem thus shows that under a realistic model of voter behavior, Arrow's theorem does not apply, and rational social choice is in fact possible if using a Condorcet method.

The majority favorite criterion is a voting system criterion that says that, if a candidate would win more than half the vote in a first-preference plurality election, that candidate should win. Equivalently, if only one candidate is ranked first by a over 50% of voters, that candidate must win. It is occasionally referred to simply as the "majority criterion", but this term is more often used to refer to Condorcet's majority-rule principle.

The Borda count electoral system can be combined with an instant-runoff procedure to create hybrid election methods that are called Nanson method and Baldwin method. Both methods are designed to satisfy the Condorcet criterion, and allow for incomplete ballots and equal rankings.

In single-winner voting system theory, the Condorcet loser criterion (CLC) is a measure for differentiating voting systems. It implies the majority loser criterion but does not imply the Condorcet winner criterion.

The reversal criterion is a voting system criterion which says that if every voter's opinions on each of the candidates is perfectly reversed, the outcome of the election should be reversed as well, i.e. the first- and last- place finishers should switch places. In other words, the results of the election should not depend arbitrarily on whether voters rank candidates from best to worst, or whether we ask them to rank candidates from worst to best.

Later-no-harm is a property of some ranked-choice voting systems, first described by Douglas Woodall. In later-no-harm systems, increasing the rating or rank of a candidate ranked below the winner of an election cannot cause a higher-ranked candidate to lose.

The Borda method or order of merit is a positional voting rule which gives each candidate a number of points equal to the number of candidates ranked below them: the lowest-ranked candidate gets 0 points, the second-lowest gets 1 point, and so on. Once all votes have been counted, the option or candidate with the most points is the winner.

Instant-runoff voting (IRV), also known as ranked-choice voting (RCV) or the alternative vote (AV), combines ranked voting together with a system for choosing winners from these rankings by repeatedly eliminating the candidate with the fewest first-place votes and reassigning their votes until only one candidate is left. It can be seen as a modified form of a runoff election or exhaustive ballot in which, after eliminating some candidates, the choice among the rest is made from already-given voter rankings rather than from a separate election. Many sources conflate this system of choosing winners with ranked-choice voting more generally, for which several other systems of choosing winners have also been used.

<span class="mw-page-title-main">Electoral system</span> Method by which voters make a choice between options

An electoral or voting system is a set of rules used to determine the results of an election. Electoral systems are used in politics to elect governments, while non-political elections may take place in business, non-profit organisations and informal organisations. These rules govern all aspects of the voting process: when elections occur, who is allowed to vote, who can stand as a candidate, how ballots are marked and cast, how the ballots are counted, how votes translate into the election outcome, limits on campaign spending, and other factors that can affect the result. Political electoral systems are defined by constitutions and electoral laws, are typically conducted by election commissions, and can use multiple types of elections for different offices.

A major branch of social choice theory is devoted to the comparison of electoral systems, otherwise known as social choice functions. Viewed from the perspective of political science, electoral systems are rules for conducting elections and determining winners from the ballots cast. From the perspective of economics, mathematics, and philosophy, a social choice function is a mathematical function that determines how a society should make choices, given a collection of individual preferences.

<span class="mw-page-title-main">Center squeeze</span> Bias of some electoral systems that favors extremists

A center squeeze is a type of spoiler effect where a majority-preferred and socially-optimal candidate is eliminated in favor of more extreme candidates in plurality-runoff methods, like the two-round and ranked-choice runoff (RCV) rules. In a center squeeze, the presence of more-extreme candidates "squeezes" a candidate trapped between them, starving them of the first-preference votes they need to survive in earlier rounds.

References

  1. ↑ Riker, William Harrison (1982). Liberalism against populism: a confrontation between the theory of democracy and the theory of social choice. Waveland Pr. pp. 29–30. ISBN   0881333670. OCLC   316034736. Ordinal utility is a measure of preferences in terms of rank orders—that is, first, second, etc. ... Cardinal utility is a measure of preferences on a scale of cardinal numbers, such as the scale from zero to one or the scale from one to ten.
  2. ↑ Hamlin, Aaron (2012-10-06). "Podcast 2012-10-06: Interview with Nobel Laureate Dr. Kenneth Arrow". The Center for Election Science. Archived from the original on 2023-06-05.

    Dr. Arrow: Well, I’m a little inclined to think that score systems where you categorize in maybe three or four classes probably (in spite of what I said about manipulation) is probably the best.

  3. ↑ Hamlin, Aaron (2012-10-06). "Podcast 2012-10-06: Interview with Nobel Laureate Dr. Kenneth Arrow". The Center for Election Science. Archived from the original on 2023-06-05.

    Dr. Arrow: Well, I’m a little inclined to think that score systems where you categorize in maybe three or four classes (in spite of what I said about manipulation) is probably the best.[...] And some of these studies have been made. In France, [Michel] Balinski has done some studies of this kind which seem to give some support to these scoring methods.

  4. ↑ Hamlin, Aaron (2012-10-06). "Podcast 2012-10-06: Interview with Nobel Laureate Dr. Kenneth Arrow". The Center for Election Science. Archived from the original on 2023-06-05.
    CES: Now, you mention that your theorem applies to preferential systems or ranking systems.
    Dr. Arrow: Yes.
    CES: But the system that you're just referring to, approval voting, falls within a class called cardinal systems. So not within ranking systems.
    Dr. Arrow: And as I said, that in effect implies more information.
  5. ↑ "Bill Status H.424: An act relating to town, city, and village elections for single-seat offices using ranked-choice voting". legislature.vermont.gov. Retrieved 2024-03-23. Condorcet winner. If a candidate is the winning candidate in every paired comparison, the candidate shall be declared the winner of the election.
  6. 1 2 George G. Szpiro, "Numbers Rule: The Vexing Mathematics of Democracy, from Plato to the Present" (2010).
  7. ↑ Colomer, Josep M. (2013-02-01). "Ramon Llull: from 'Ars electionis' to social choice theory". Social Choice and Welfare. 40 (2): 317–328. doi:10.1007/s00355-011-0598-2. ISSN   1432-217X.
  8. ↑ Nanson, E. J. (1882). "Methods of election: Ware's Method". Transactions and Proceedings of the Royal Society of Victoria. 19: 206. The method was, however, mentioned by Condorcet, but only to be condemned.
  9. ↑ Condorcet, Jean-Antoine-Nicolas de Caritat (1788). "On the Constitution and the Functions of Provincial Assemblies". Complete Works of Condorcet (in French). Vol. 13 (published 1804). p. 243. En effet, lorsqu'il y a plus de trois concurrents, le véritable vœu de la pluralité peut être pour un candidat qui n'ait eu aucune des voix dans le premier scrutin.
  10. ↑ Duncan Black, "On the Rationale of Group Decision-making" (1948).
  11. ↑ Arrow, Kenneth Joseph Arrow (1963). Social Choice and Individual Values (PDF). Yale University Press. ISBN   978-0300013641. Archived (PDF) from the original on 2022-10-09.
  12. ↑ Farrell and McAllister, The Australian Electoral System, p. 17
  13. ↑ "Ranked Choice Voting in Maine". legislature.maine.gov. State of Maine. 2022-08-23. Retrieved 2022-11-20.
  14. ↑ Piper, Kelsey (2020-11-19). "Alaska voters adopt ranked-choice voting in ballot initiative". vox.com. Vox Media. Retrieved 2022-11-20.
  15. ↑ "North to the Future: Alaska's Ranked Choice Voting System is Praised and Criticized Nationally". Alaska Public Media.
  16. ↑ "Ranked Choice Voting in Maine". Maine State Legislature. Retrieved 21 October 2021.
  17. ↑ "Alaska Better Elections Implementation". Alaska Division of Elections. Retrieved 21 October 2021.
  18. ↑ "New Zealand Cities Voting to Implement Ranked Choice Voting". 19 September 2017.
  19. ↑ Weber, Robert J. (September 1978). "Comparison of Public Choice Systems". Cowles Foundation Discussion Papers. Cowles Foundation for Research in Economics: 16, 38, 62. No. 498.
  20. ↑ T. N. Tideman and F. Plassman, "Modeling the Outcomes of Vote-Casting in Actual Elections" (2012).
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