Ranked voting

Ranked voting is any voting system that uses voters' rankings of candidates to choose a single winner or multiple winners. More formally, a ranked vote system depends only on voters' order of preference of the candidates.

Ranked voting systems vary dramatically in how preferences are tabulated and counted, which gives them very different properties. In instant-runoff voting (IRV) and the single transferable vote system (STV), lower preferences are used as contingencies (back-up preferences) and are only applied when all higher-ranked preferences on a ballot have been eliminated or when the vote has been cast for a candidate who has been elected and surplus votes need to be transferred. Ranked votes of this type do not suffer the problem that a marked lower preference may be used against a voter's higher marked preference.

Some ranked vote systems use ranks as weights; these systems are called positional voting. In the Borda method, the 1st, 2nd, 3rd... candidates on each ballot receive 1, 2, 3... points, and the candidate with the fewest points is elected. Thus intensity of preference is assumed to be at ratios of 1 to 2, 2 to 3, etc.

In the United States and Australia, the terms ranked-choice voting and preferential voting, respectively, almost always refer to instant-runoff voting; however, because these terms have also been used to mean ranked systems in general, many social choice theorists recommend the use of the term instant-runoff voting in contexts where confusion might arise.

Ranked votes do not incorporate any information about intensity of preferences. Furthermore, common implementations do not account for equality of preference among two or more candidates.

Ranked voting systems of the instant-runoff voting type and the Borda count type are contrasted with rated voting methods, which allow voters to indicate how strongly they support different candidates (e.g. on a scale from 0 to 10). Ranked vote systems produce more information than X voting systems such as first-past-the-post voting. Rated voting systems produce more information than ordinal ballots; as a result, some common results like Arrow's theorem do not directly apply to them.

Some ranked voting systems require the voter rank a set number of candidates. Others, such as optional preferential voting, allow the voter full liberty as to how many candidates they rank. Under STV or IRV, not all rankings are used in any case.