Preferential voting

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Term preferential voting (also known as the preference voting) has several different meanings:

(1) A ranked ballot or preferential voting system is a type of voting system in which each voter casts their vote by ranking candidates in order of preference.

Voting systems which use a ranked ballot include:

(2) Preferential voting is a synonym for instant-runoff voting, especially in Australia, where such ballots are actually in use in elections. See Australian electoral system.

(3) In Europe, preferential voting denotes what is in United States known as the Open List Proportional Representation (Open list PR). It is a voting system giving a voter an option to vote for one of the party lists and then also express a preference for one of the candidates of this list.

(4) Often term preferential voting is used for any kind of intraparty preference.

Ballot design or voting machine instructions are particularly important in such systems, as each voter is expected to express a rather complex set of tolerances or preferences in each vote.

Ballot variations

  • Column marks: Optical scanner ballots use the ballot with column voting with ovals.
    • Column rank ballots have limits rankings due primarily to available paper space. For example the image below is limited to three rankings.
  • Write numbers: Hand-written numeric rankings are more compact to vote and easier to hand count.
  • Write names: Hand-written names as a list from first to last preference.
  • Touch screen: A slightly different category of voting is a computer Touch screen could also be used, asking voters their first, second, etc preferences, and showing the selections so far and remaining choices, allowing selections to be removed if the voter makes a mistake or changes her mind during voting. Some people want touch screen voting to print a paper ballot at the end as a hardcopy backup.

Equal rankings

Another variation of ranked ballots is to allow a voter to give multiple candidates the same ranking. This allows voters to cast range voting style ballots. Ways of dealing with such ballots include:

  • Convert the ballot into a fully-ranked one by breaking ties at random.
  • Use fractional votes. If N candidates are tied on a ballot, each candidate receives 1/N of a first-place vote, 1/N of a second-place vote, etc.
  • In most Condorcet methods, the defeat strength of X over Y can be computed from the X>Y and Y>X votes, while simply ignoring X=Y votes.

Scope for corruption

A potential problem with preferential votes is that they can be used to undermine a secret ballot, and thus enable corruption by vote buying. If there are enough candidates then the number of possible voting patterns may be much larger than the number of voters, and it then becomes possible to use early preferences to vote for the desired candidates and then to use later preferences to identify the voter to the person who has purchased the vote and looks at the ballot papers.

As an example, in the Irish general election of 2002, the electronic votes were published for the Dublin North constituency. There were 17 candidates, allowing more than 966 million million possible patterns of preferences, but there were fewer than 44,000 votes cast. The most common pattern (for three of the candidates in a particular order) was chosen by 800 voters, and more than 16,000 patterns were chosen by just one voter each.

One way to avoid this possibility for buying a vote and confirming it has been cast as specified is to prevent partisan observers from systematically viewing individual voter's preferences.

How to Vote Cards and "Above the line" voting

In Australia, which uses preferential voting for both houses, candidates hand out at the entrance to Polling Stations "How to Vote Cards", which advise voters how best to fill in their ballots to support that candidate, and any cross preference deals they may have arranged with other candidates. These HTVC cards are voluntary, and no voter is obliged to do so, but high proportion are happy to do so. In elections for the upper houses, which use proportional representation as well as preferential voting, it may be daunting to have to fill in 70 boxes - preferences are compulsory. To ease this onerous task, "Above the line" voting, allows the voter to choose one party or group, and all the remaing squares are deemed to be filled in according to a registered party ticket. About 95% of voters choose to use this method.

Parliamentary procedure

Recent editions of Robert's Rules of Order mention preferential voting. The book notes, "While it is more complicated than other methods of voting in common use and is not a substitute for the normal procedure of repeated balloting until a majority is obtained, preferential voting is especially fair and useful in an election by mail if it is impractical to take more than one ballot" (RONR [10th ed.], p. 411, l. 23). Rather than discuss Condorcet, the Borda count, and other lesser-known variations, the book simply states, "Preferential voting has many variations. One method is described here by way of illustration," and proceeds with a description of instant runoff voting.

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