Voting system

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Voting systems or election methods are methods for groups of people to select one or more options from many, taking into account the individual preferences of the group members. Voting is often seen as the defining feature of democracy, and is best known for its use in elections — but it can also be used to award prizes, to select between different plans of action, or as a means for computer programs to evaluate which solution is best for a complex problem.

A key property of voting systems is that, because they are algorithms, they must be formally defined. Consensus, for example, which is sometimes put forward as a voting system, is more properly a broad way of working with others, analogous to democracy or anarchy.

Aspects of voting systems

The ballot

Different voting systems have different forms for allowing the individual to express their tolerances or preferences. In ranked ballot or "preference" voting systems, like Instant-runoff voting, the Borda count, or a Condorcet method, voters order the list of options from most to least preferred. In range voting, voters rate each option separately. In first-past-the-post (also known as plurality voting), voters select only one option, while in approval voting, they can select as many as they want. In voting systems that allow plumping, like cumulative voting, voters may vote for the same candidate multiple times.

District (constituency) size

A voting system may select only one option (usually a candidate, but also an option that represents a decision), in which case it is called a "single winner system", or it may select multiple options, for example candidates to fill an assembly or alternative possible decisions on the measure the ballot posed.

Some countries, like Israel, fill their entire parliament using a single multiple-winner district (constituency), while others, like Ireland or Belgium, break up their national elections into smaller, multiple-winner districts, and yet others, like the United States or the United Kingdom, hold only single-winner elections. Some systems, like the Additional member system, embed smaller districts within larger ones.

Party-list systems

In party-list proportional representation systems, candidates can be aligned with, or nominated by, parties, and the party's list of candidates plays a functional role within the system. These parties may in turn be aligned with other parties, to form coalitions, which can play roles beyond those played by the party. These systems are designed to ensure proportional representation, the idea that the candidates selected from a given party (or, in non-party-list systems, informal grouping) should be in proportion to the votes cast for that party. Some of these systems, however, have election thresholds--minimum numbers of votes cast for a party to win any seats. The purpose of an election threshold is generally to keep very small parties from participating in a parliament, in order to maintain stability of governments.

None of the above option

In some voting systems, voters may choose to select none of the candidates (or poll options), by voting for a "None of the above" option. If this option wins, the election fails, all candidates or poll options are excluded from a subsequent election.

Write-in candidate - poll option

Some elections allow voters to write in the name of a person (or of the poll option) not on the ballot as their candidate (or as a poll option). Write-in candidates (poll options) rarely win and votes are often cast for ineligible people or fictional characters. This happens because write-in poll options or candidates are not visible to other voters. This is not usually an issue in the case of an e-voting system, where new write-in poll options or candidates can be made visible as the election takes place. Alternatively, some locations require write-in candidates or poll options to be registered before the election.

Criteria in evaluating voting systems

Various criteria are used in evaluating voting systems. However, it is impossible for one voting system to pass all criteria in common use. For example, Arrow's impossibility theorem demonstrates that the following criteria are mutually contradictory:

  • The voting system should always give a result
  • If a voter improves the ranking of a particular option, that option should not be disadvantaged (monotonicity criterion)
  • Removing a candidate should not change the winner of an election unless that candidate is the winner (independence of irrelevant alternatives)
  • Every possible outcome should be achievable
  • Non-dictatorship (i.e. more than one person's vote matters)

Other criteria which have been used to judge voting systems include:

Voting systems can be abstracted as mathematical functions that select between choices based on the utility of each option for each voter. This greatly resembles a social welfare function as studied in welfare economics and many of the same considerations can be studied. For aspects such as simplicity, dispute, and fraud, the practical implementation is far more important than the abstract function. However, the choice of abstract function puts some constraints on the implementation. For instance, certain voting systems such as First Past the Post, Schulze, or Borda Count can be tallied in one distributed step, others such as IRV require centralization, and others such as multi-round runoff require multiple polling rounds.

List of systems

Single Winner Systems

Single Winner systems can be classified by ballot type:

  1. Binary voting A valid vote can only give a yes or nothing to a given candidate.
  2. Ranked voting A valid vote can rank candidates 1,2,3... (Tied rankings are permitted in some methods but not others)
  3. Rated voting A valid vote allows independent numerical values to be associated with each candidate. (The set of valid values is limited.)

They can also be classified on how many times votes can be counted. Methods like Plurality, Borda, and Approval with single counting rounds are simpler since voters can be sure to know how their votes will be applied.

Binary voting methods

  • First-past-the-post (also called Plurality or Relative Majority or Winner-Take-All) - vote for at most one candidate. Most votes wins, even if this is less than a majority.
  • Runoff systems
    • Runoff voting - Two Round System - if no majority, hold a new election with only the top two candidates. This system is used for most single-winner elections in France.
    • Elimination runoff - if no majority, hold a new election with the weakest candidate eliminated. Repeat until there is a majoirty.
    • Exhaustive runoff - no eliminations, repeat balloting until there is a majority. Common in committees.
  • Approval voting (AV) - Voters may for vote for as many candidates as they like. Candidate with most votes wins. Sometimes considered a version of Cardinal Rankings (see below) with a point range of [0,1]
  • Random ballot - May also be used for multiwinner elections, or as a tiebreaker for other methods

Ranked Voting methods

  • Tied rankings not permitted
  • Tied rankings permitted
    • Condorcet method, actually several families of systems that satisfy Condorcet's criterion:
      • Ranked Pairs (RP) and variants such as Maximize Affirmed Majorities and Maximum Majority Voting
      • Schulze, which is also known as "Beatpath Method" or "Cloneproof Schwartz Sequential Dropping"
      • Copeland's method
      • Other names for Condorcet-compliant methods:
        • VOTE-123: another name for Condorcet methods, stands for Virtual One-on-one Tournament Elections using 1st, 2nd, & 3rd choices
        • Majority voting or Maximum Majority voting: another term often used for Condorcet methods
    • Bucklin voting: approval runoff; voters vote for more candidates each round until a candidate reaches a majority

Rated Voting methods

  • Cardinal Ratings (CR) (Also called range voting) - voters give whole number points (example 1-10) to each candidate, totaled in single round
  • Majority Choice Approval (MCA) - like Approval but with range [0,2] and multiple counting rounds.
  • Rated Ballots may also be used for ranked voting methods, in cases where tied rankings are allowed.

Single Winner Variations

Automatic Equal Ranking Line Option (AERLO) 
A voter may mark a line in his/her ranking, meaning that if no one above that line wins, then that voter wants to promote to 1st place all of his/her above-line candidates and have a recount. (In pairwise-count methods the promotion only takes place if, additionally, there's a circular tie containing above-line and below-line candidates).
Automatic Truncation Line Option (ATLO) 
A voter may mark a line in his/her ranking, meaning that if no one above the line wins, then that voter wants to drop from his/her ranking all of his/her below-line candidates and have a recount. (In pairwise-count methods the dropping only takes place if, additionally, there's a circular tie containing above-line and below-line candidates).

Multiple Winner Systems

Related terminology

voting strategy 
Any way of voting, when it's discussed in terms of its possible or intended affect on the outcome.
strategic or tactical voting 
When a voter self-consciously marks a ballot in a manner different than their actual preferences, in the hope of optimizing the outcome. (While the adjectives 'strategic' and 'tactical' usually have nearly opposite meanings when used to describe other things, in this case, they commonly both have the meaning given here.)

Famous theoreticians of voting systems

See also

External links

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