Ranked voting systems

Preferent or rank voting describes certain voting systems in which voters rank outcomes in a hierarchy on the ordinal scale (ordinal voting systems). 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. There is likewise no consensus among academics or public servants as to the best electoral system.

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.

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, so "Condorcet efficiency" is important when evaluating different methods of preferential voting. This choice is also the one that would win every two-way contest against every other alternative.

Another criterion used to gauge the effectiveness of a preferential voting system is its ability to withstand manipulative voting strategies, 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 and the complexity of the mechanism necessary.

...
Wikipedia