Ranked voting · Lablypedia

Ranked voting systems vary dramatically in how preferences are tabulated and counted. This gives them different properties with respect to satisfying various voting groups and adherence to mathematical rules.

In instant-runoff voting (IRV) and the single transferable vote system (STV), lower preferences are used as contingencies and are only applied when all candidates marked as higher-ranked preferences on a ballot have been eliminated or when the vote has been cast for a candidate who has been elected and surplus votes need to be transferred. Ranked votes of this type do not suffer the problem that a marked lower preference may be used against a voter's higher marked preference.

Some ranked vote systems do not use ranking to transfer votes but instead use ranks as weights. These systems are called positional voting. In the Borda method, the least-favored candidates on each ballot receive fewer points; the most-favored receive more points. The points on each ballot for each candidate are added together, and the candidate with the most points is elected.

In the United States and Australia, the terms ranked-choice voting and preferential voting, respectively, almost always refer to instant-runoff voting; however, because these terms have also been used to mean ranked systems in general, many social choice theorists recommend the use of the term instant-runoff voting in contexts where confusion might arise.

Ranked votes do not incorporate any information about intensity of preferences. Furthermore, common implementations do not account for equality of preference among two or more candidates.

Ranked voting systems that use ordinal numbers (1, 2, 3, etc.) such as IRV, STV and the Borda method are contrasted with rated voting methods, which allow voters to indicate how strongly they support candidates (e.g. on a scale from 0 to 10). Ranked vote systems using ordinal numbers produce more information than X voting systems such as first-past-the-post voting. Rated voting systems produce more information than systems that use ordinal ballots; as a result, some common results, like Arrow's theorem, do not directly apply to them.

Some ranked voting systems require the voter rank a set number of candidates. Those that use optional preferential voting allow the voter full liberty as to how many candidates they rank. Under STV or IRV, not all rankings are used in any case.

History of ranked voting theory and its adoption

Plurality voting is the most common voting system and has been in widespread use since the earliest democracies. As plurality voting has exhibited weaknesses from its start, especially as soon as a third party joins the race, some social scientists began to recommend transferable votes (facilitated by contingent ranked ballots) to reduce the incidence of wasted votes and unrepresentative election results.

A form of the single transferable vote (STV) system was invented by Carl Andræ in Denmark, where it was used briefly before being abandoned in favour of open list proportional representation (open list PR). STV still carried on in Denmark until 1953 for indirect election of the upper house.

At approximately the same time, the STV system was independently devised by British lawyer Thomas Hare, whose writings soon spread the method throughout the British Empire. Tasmania used STV (called the Hare system) in government elections in the 1890s. STV began permanent and wider adoption throughout Australia beginning in 1907 and throughout the 1910s. The STV system, using contingent ranked votes, has been adopted in Ireland, South Africa, Malta, and approximately 40 cities in the United States and Canada. The single transferable vote system has also been used to elect legislators in Canada, South Africa, India and Nepal.

The earliest known proposals for a ranked voting system 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. Llull presented the idea in his 1299 treatise Ars Electionis, which was discussed by Nicholas of Cusa in the fifteenth century.

A second wave of analysis began when Jean-Charles de Borda published a paper in 1781, advocating the Borda count, which he called the "order of merit". This methodology drew criticism from the Marquis de Condorcet, who pointed out the Borda method did not always reflect group preferences because it was vulnerable to spoiler effects and did not always elect the majority-preferred candidate. Condorcet developed his own election method, which was similar to the earlier Copeland's method.

In the 1700s, Condorcet pointed out the weakness of systems that eliminate a candidate out of hand after the first round, such as the two-round system or instant-runoff voting (IRV). He rejected them as having the weakness that they might eliminate a candidate who is the actual preference of most voters. In 1864, Edward J. Nanson, while noting Condorcet's concerns, said the IRV method is "a perfectly feasible and practicable one for elections on any scale".

Interest in ranked voting continued throughout the 19th century. The concept of ranked voting as merely a series of alternate preferences, as opposed to weighted rankings, was formulated by Carl Andræ in his version of the single transferable vote election system. This system was adopted and put into use in his country, Denmark, in 1855. This used contingent ranked votes to transfer votes of elected candidates, but it did not include transfers of votes from eliminated candidates.

About the same time in unrelated research, British reformer Thomas Hare formulated an early form of single transferable voting, again, like Andræ, using rankings as contingent preferences to be used only if needed.

Massachusetts Institute of Technology (MIT) professor W. R. Ware held a demonstration single transferable vote election in 1871 by having 150 MIT students elect four English authors as their favorites. The field of candidates held the names of 35 authors in the first count and eventually concentrated a large proportion of the votes behind just four. Ware went on to surmise how a casual vacancy might be handled and found that countback would produce a winner. This proved that contingent ranked transferable votes (instant-runoff voting) could be used to produce a single winner, despite the qualms of Condorcet and others.

In the first half of the 20th century, single transferable voting (multi-seat contests using contingent ranked transferable votes) found common use in cities and was used to elect legislators in North America, Ireland and other parts of the English-speaking world, with single-winner versions, alternative voting (now known as instant-runoff voting) and the contingent vote system (or its variant known as supplementary voting) being its companions for single-winner contests of mayors and others.

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

Resurgence of ranked voting in the 2020s

In the early 2020s, the use of contingent ranked votes saw a comeback in the United States. STV, for a time used only in Cambridge, Massachusetts, was adopted by Portland, Oregon, and several other American cities beginning in 2022.

Single-winner ranked voting (specifically instant-runoff voting) is used to elect politicians in the states of Maine and Alaska. In November 2016, the voters of Maine narrowly passed Question 5, approving instant-runoff voting for all elections. This was first put to use in 2018, marking the inaugural use of ranked votes in a statewide election in the United States. (Maine's statewide vote tally was used to determine the two state-wide electoral college seats.)

In November 2020, Alaska voters passed Measure 2, bringing instant-runoff voting into use from 2022. 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 Mary Peltola, the ultimate winner in the election. However, a referendum on the issue in 2024 saw a narrow majority in favour of retention of IRV.

Some local elections in New Zealand use the multi-winner single transferable vote system. STV is also used to elect local authorities in Scotland and Ireland.

Nauru uses a rank-weighted positional method called the Dowdall system.

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Equal-ranked ballots

In voting with a ranked ballot, a tied or equal-rank ballot is one where two or more candidates receive the same rank or rating. In instant runoff and first-past-the-post, such ballots are generally rejected as the intent of the voter is unclear. However, in social choice theory, some election systems assume equal-ranked ballots are "split" evenly between the equal-ranked candidates (e.g. in a two-way tie, each candidate receives half a vote). The Borda count and the Condorcet method can handle equal-rank ballots differently than simply splitting a vote. Different rules produce different mathematical properties and behaviors, particularly under strategic voting.

Truncated ballots

When a voter does not rank all the candidates, the ballot is called truncated or partial. In IRV and STV, a truncated ballot may become exhausted once all candidates it ranks have been eliminated. Voters may be forced to truncate in a jurisdiction that limits the number of allowed rankings or may truncate voluntarily because they lack information about lesser‑known candidates or are indifferent toward them. Truncation can also be used deliberately as a strategy, with the most extreme case being bullet voting. This can give rise to a Burr dilemma in which a voter helps a less‑preferred candidate by withholding later preferences.

Theoretical foundations of ranked voting

Majority-rule

Concepts formulated by the Marquis de Condorcet in the 18th century continue to be important in the study of elections. One of these concepts is the Condorcet winner, a candidate who would win against any other candidate in a two-way race. 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.

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.

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.

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.

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 weighted-rank 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. Also, 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 voting

Instant-runoff voting, often conflated with ranked-choice voting in general, is a contingent ranked-vote voting method that recursively eliminates the successive plurality loser during rounds of counting and transfers their votes, if possible, until one candidate has the majority of the votes. 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. Instant-runoff voting does not fulfill the Condorcet winner criterion.

Single transferable voting

Single transferable voting is a contingent ranked-vote election method that elects multiple members. It elects any candidates who achieve quota, and if necessary recursively eliminates the plurality loser at various stages of the vote count. It transfers the votes of eliminated candidates and surplus votes of winners until enough are elected by quota or by still being in the running when the field of candidates is thinned to the number of remaining open seats. Because elected members are elected with the same or about the same number of votes, each party popular enough for representation receives a number of seats appropriate to the vote tallies of its candidates. The transfers reduce waste to about one quota – which in a five-seat district is about 17 percent of valid votes; in districts with more members than five, the waste is smaller. All but one quota of votes approximately are used to actually elect someone in the district so the percentage of effective votes is dependably about 80 to 90 percent of valid votes in the district.

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 the minimax Condorcet method, 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).

How-To-Vote cards

In Australian Commonwealth and state elections, voters are handed How-To-Vote cards (HTVC) which recommend how best to support a chosen candidate. In South Australia, the Electoral Commission publishes a combined HTVC for all candidates which saves time and money. This also benefits minor parties and independents who are unable to staff all polling booths.