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.
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 rank the list of options in their prefered order. In range voting, voters assign numeric ratings to each option. In first-past-the-post, 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.
A voting system may select only one option, in which case it is called a "single winner system", or it may select multiple options, for example to fill a parliament. Some places, like Israel, fill their entire parliament using a single multiple-winner district, 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.
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.
In some voting systems, voters may choose to select none of the candidates, by voting for a "None of the above" option.
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:
Single Winner Systems
Aspects of voting systems
The ballot
District size
Party-list systems
None of the above
Write-in candidate
Some elections allow voters to write in the name of a person not on the ballot as their candidate. Write-in candidates rarely win and votes are often cast for ineligible people or fictional characters. Some locations require write-in candidates to be registered as candidates before the election. Criteria in evaluating voting systems
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.List of systems
Multiple Winner Systems
Mixed Systems
Famous theoreticians of voting systems
See also
External links