Voting system

Single-winner methods

Single-winner systems can be classified based on their ballot type. Binary voting systems are those in which a voter either votes or doesn't vote for a given candidate. In ranked voting systems, each voter ranks the candidates in order of preference. In rated voting systems, voters give a score to each candidate.

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Binary voting methods

The most prevalent single-winner voting method, by far, is plurality (also called "first-past-the-post", "relative majority", or "winner-take-all"), in which each voter votes for one choice, and the choice that receives the most votes wins, even if it receives less than a majority of votes.

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Approval voting is another binary voting method, where voters may vote for as many candidates as they like. The choice that receives the most approval votes wins.

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Runoff methods hold multiple rounds of plurality voting to ensure that the winner is elected by a majority. Top-two runoff voting, the second most common method used in elections, holds a runoff election between the top two options if there is no majority. In elimination runoff elections, the weakest candidate is eliminated until there is a majority. In an exhaustive runoff election, no candidates are eliminated, so voting is simply repeated until there is a majority.

Related Topics:
Runoff methods - Top-two runoff - Elimination runoff - Exhaustive runoff

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Random ballot is a method in which each voter votes for one option, and about a selected at random to determine the winner. This is mostly used as a tiebreaker for other methods.

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Ranked voting methods

Also known as preferential voting methods, these methods allow each voter to rank the candidates in order of preference. Often it is not necessary to rank all the candidates: unranked candidates are usually considered to be tied for last place. Some of these methods also allow voters to give multiple candidates the same ranking.

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The most common ranked voting method is instant-runoff voting (IRV), also known as "alternative vote" or simply "preferential voting", which uses voters' preferences to simulate an elimination runoff election without multiple rounds of voting. As the votes are tallied, the option with the fewest first-place votes is eliminated until there is a majority. Supplementary voting is a variation on IRV with only two rankings and two rounds. Coombs' method is a rarely-used variation that eliminates the candidate with the most last-place votes.

Related Topics:
Instant-runoff voting - Supplementary voting - Coombs' method

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The Borda count is a simple ranked voting method in which the options receive points based on their position on each ballot. A class of similar methods are called positional voting systems.

Related Topics:
Borda count - Positional voting system

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Bucklin voting is a runoff method based on approval voting, in which voters approve successively more candidates until one is approved by a majority.

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Condorcet methods

Condorcet methods, or pairwise methods, are a class of ranked voting methods that meet the Condorcet criterion. These methods compare every option pairwise with every other option, and an option that defeats every other option is the winner. An option defeats another option if a majority of voters rank it higher on their ballot than the other option.

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These methods are often referred to collectively as the Condorcet method, because the Condorcet criterion ensures that they all give the same result in most elections. The differences occur in situations where no option is undefeated, meaning that there exists a cycle of options that defeat each other.

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A simple version of Condorcet is Minimax. If no option is undefeated, the option that is defeated by the fewest votes in its worst defeat wins. Another simple method is Copeland's method, in which the winner is the option that wins the most pairwise contests.

Related Topics:
Minimax - Copeland's method

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The Schulze method (also known as "cloneproof Schwartz sequential dropping" or the "beatpath method") and Ranked Pairs are two recently-designed Condorcet methods that satisfy a large number of voting method criteria.

Related Topics:
Schulze method - Ranked Pairs - Voting method criteria

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Rated voting methods

Rated ballots allow even more flexibility than ranked ballots, but few methods are designed to use them. Each voter gives a score to each option; the allowable scores could be numeric (for example, from 0 to 100) or could be "grades" like A/B/C/D/F.

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In range voting, voters give numeric ratings to each option, and the option with the highest total score wins. Approval voting can be seen as an instance of range voting where the allowable ratings are 0 and 1. Cumulative voting restricts the range differently by requiring the points on a ballot to add up to a certain total.

Related Topics:
Range voting - Cumulative voting

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Rated ballots may also be used for ranked voting methods, in systems where tied rankings are allowed.

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