Ranked voting system

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Preferential voting or rank voting describes certain voting systems in which voters rank outcomes in a hierarchy on the ordinal scale. When choosing between more than two options, preferential voting systems provide a number of advantages over first-past-the-post voting (also called plurality voting). This does not mean that preferential voting is the best system; Arrow's impossibility theorem proves that no preferential method can simultaneously obtain all properties desirable in a voting system.^{[Mankiw 1][1]} There is likewise no consensus among academics or public servants as to the best electoral system.^[2]

There are many types of preferential voting, but only instant-runoff voting and single transferable vote are being used in governmental elections. Instant runoff voting is employed in Australia at the state and federal levels, in Ireland for its presidential elections, and by some cities in the United States, United Kingdom, and New Zealand. The single transferable vote is used for national elections in the Republic of Ireland and Malta, the Australian Senate, for regional and local elections in Northern Ireland, for all local elections in Scotland, and for some local elections in New Zealand and the United States.

Variety of systems

There are many preferential voting systems, so it is sometimes difficult to distinguish between them.

Selection of the Condorcet winner is generally considered by psephologists as the ideal election outcome,^[3] so "Condorcet efficiency" is important when evaluating different methods of preferential voting.^[4] This choice is also the one that would win every two-way contest against every other alternative.^{[Mankiw 2]}

Another criterion used to gauge the effectiveness of a preferential voting system is its ability to withstand manipulative voting strategies,^[5] when voters cast ballots that do not reflect their preferences in the hope of electing their first choice. This can be rated on at least two dimensions—the number of voters needed to game the system^[6] and the complexity of the mechanism necessary.

Instant-runoff voting

Used in national elections in Australia, this system is said to simulate a series of runoff elections. If no candidate is the first choice of more than half of the voters, then all votes cast for the candidate with the lowest number of first choices are redistributed to the remaining candidates based on who is ranked next on each ballot.^[7] If this does not result in any candidate receiving a majority, further rounds of redistribution occur.^[7] Or, in other words, "[...] voters would rank their first, second and subsequent choices on the ballot. The candidate with the fewest votes would be dropped and his or her supporters’ second choices would be counted and so on until one candidate emerged with more than 50 per cent."^[8]

This method is thought to be resistant to manipulative voting as the only strategies that work against it require voters to highly rank choices they actually want to see lose.^{[G&F 1]} At the same time, this system fails the monotonicity criterion, where ranking a candidate higher can lessen the chances he or she will be elected. Additionally, instant-runoff voting has a lower Condorcet efficiency than similar systems when there are more than four choices.^{[G&F 2]}

Borda count

In the Borda count, ballots are counted by assigning a point value to each place in each voter's ranking of the candidates, and the choice with the largest number of points overall is elected.^{[Mankiw 1]} This method is named after its inventor, French mathematician Jean-Charles de Borda.^{[Mankiw 1]} Instead of selecting a Condorcet winner, this system may select a choice that reflects an average of the preferences of the constituency.

This system suffers from the fact that the outcome it selects is dependent on the other choices present. That is, the Borda count does not exhibit independence of irrelevant alternatives^{[Mankiw 1]} or independence of clones. The Borda count can be easily manipulated by adding candidates, called clones, whose views are identical to the preferred candidate's. An example of this strategy can be seen in Kiribati's 1991 presidential nomination contest.^[9]

Single transferable vote

This is one of the preferential voting systems most used by countries and states.^{[notes 1]} It uses multi-member constituencies. Any candidates that achieve the number of votes required for election (the "quota") are elected and their surplus votes are redistributed to the voter's next choice candidate.^{[CEPPS 1]} Once this is done, if not all places have been filled then the candidate with the lowest number of votes is eliminated, and their votes are also redistributed to the voter's next choice. This whole process is repeated until all seats are filled. This method is also called the Hare-Clark system.^{[CEPPS 1]}

Uniqueness of votes

If there are a large number of candidates, which is quite common in single transferable vote elections, then it is likely that many preference voting patterns will be unique to individual voters.^[10][11] For example, in the Irish general election, 2002, the electronic votes were published for the Dublin North constituency.^[12] There were 12 candidates and almost 44,000 votes cast. The most common pattern (for the three candidates from one party in a particular order) was chosen by only 800 voters, and more than 16,000 patterns were chosen by just one voter each.

The number of possible complete rankings with no ties is the factorial of the number of candidates, N, but with ties it is equal to the corresponding ordered Bell number and is asymptotic to

.^[13]

In the case common to instant-runoff voting in which no ties are allowed, except for unranked candidates who are tied for last place, the number of possible rankings for N candidates is precisely

.^[14]

Use by polities

Countries

Federated states

International organizations

Municipalities

See also

• FairVote
• Donkey vote

External links

• Brent, Peter, A Short History of Preferential Voting (in Australia) Mumble Blog, The Australian. 17 April 2011.

References

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2. ↑
3. ↑