Single transferable vote
The single transferable vote (STV), sometimes known as proportional ranked choice voting (P-RCV), is a multi-winner electoral system in which each voter casts a single vote in the form of a ranked-choice ballot. Voters have the option to rank candidates, and their vote may be transferred according to alternate preferences if their preferred candidate is eliminated or elected with surplus votes, so that their vote is used to elect someone they prefer over others in the running. STV aims to approach proportional representation based on votes cast in the district where it is used, so that each vote is worth about the same as another.
For the single-winner version of STV, see Instant-runoff voting.
Under STV, no one party or voting bloc can take all the seats in a district unless the number of seats in the district is very small or almost all the votes cast are cast for one party's candidates (which is seldom the case). This makes it different from other commonly used candidate-based systems. In winner-take-all or plurality systems – such as first-past-the-post (FPTP), instant-runoff voting (IRV), block voting, and ranked-vote block voting – one party or voting bloc can take all the seats in a district.
The key to STV's approximation of proportionality is that each voter effectively only casts a single vote in a district contest electing multiple winners, while the ranked ballots (and sufficiently large districts) allow the results to approach proportionality. The use of a quota means that, for the most part, each successful candidate is elected with the same number of votes. This equality produces fairness in the particular sense that a party taking twice as many votes as another party will generally take twice the number of seats compared to that other party.
Under STV, winners are elected in a multi-member constituency (district) or at-large, also in a multiple-winner contest. Every sizeable group within the district wins at least one seat: the more seats the district has, the smaller the size of the group needed to elect a member. In this way, STV provides approximately proportional representation, ensuring that substantial minority factions have some representation.
STV is distinguished from plurality voting systems – like FPTP, plurality block voting and the single non-transferable vote (SNTV) – by the fact that a vote may be transferable under STV but is not under the other systems. STV reduces the number of "wasted" votes, votes which are cast for unsuccessful candidates, by electing multiple representatives for a district. As well, a vote initially cast for an unelectable candidate may be transferred to a different candidate who is elected. Additionally, surplus votes collected by successful candidates are transferred to aid other candidates, preventing waste caused by successful candidates receiving votes over and above those actually needed to secure the seat.
An important characteristic of STV is that it enables votes to be cast for individual candidates rather than for parties. Party lists are therefore not needed (as opposed to many other proportional electoral systems); it is the voters who create their own ordered list of candidates.
Related voting systems[edit]
Instant-runoff voting (IRV) is the single-winner analogue of STV. It is also called "single-winner ranked-choice voting". Its goal is representation of a majority of the voters in a district by a single official, as opposed to STV's goals of not only the representation of a majority of voters through the election of multiple officials but also of proportional representation of all the substantial voting blocks in the district.
Single non-transferable vote (SNTV) produces much the same representation as STV, without the work and complication of preferential ballots and vote transfers. Single voting in a multiple-member district produces mixed roughly proportional representation, which STV's vote transfers sometimes does not alter. (An example was the election of Edmonton, Alberta, MLAs through STV in 1930. The winners were the same under STV as would have been elected under SNTV.)
The spare vote[3] is a version of single transferable voting applied to the ranking of parties, first proposed for elections in Germany in 2013.[4] The spare vote system includes the step of transferring the votes of eliminated choices to the next-indicated choice, but it does not transfer surplus votes.
The mixed ballot transferable vote (MBTV) is a mixed version of STV, where voters may rank both candidates and parties, even both interchangeably, depending on the ballot type, but must choose at least a local (district) candidate (1st preference) and a national list (2nd preference). The list preferences are used if the vote is unused in the district election, which may use FPTP, IRV or STV rules; in the STV case, the vote is transferred to another tier in favour of the chosen party list. (This is in contrast to the mixed single vote, which is currently used in Hungary, where voters may not define a separate party-list preference and do not cast preferential votes.)
Hare–Clark is a variant of PR-STV used in Tasmania and the Australian Capital Territory.[5][6]
Indirect single transferable voting is a non-ranked-vote version of STV. Single voting in a multi-seat district is retained. Voters do not mark their ballots with rankings, but votes are transferred, as needed, based on the eliminated or elected candidate's pre-set instructions. This is a useful system to achieve many of the benefits of STV in districts where it is difficult to collect all the ballots in one central place to conduct STV transfers or where X voting is preferred over ranked voting due to voters' inability or disinterest in ranking candidates. Once known as the Gove system, or the schedule system of PR, it was invented by Massachusetts legislator William H. Gove of Salem and Archibald E. Dobbs of Ireland, author of Representative Reform for Ireland (1879).[7][8] (STV with group ticket Voting also conducts transfers without reference to alternate preferences marked by voters.) In 1884, Charles L. Dodgson (Lewis Caroll) argued for a proportional representation system based on multi-member districts similar to indirect STV, with each voter casting only a single vote, quotas as minimum requirements to take seats, and votes transferable by candidates through what is now called liquid democracy. The difference from "indirect STV" is that under liquid democracy, candidates and members may transfer votes after the votes are cast to build coalitions; they do not have to publish their list beforehand.
The modified d'Hondt electoral system[9] is a variant of STV, where an electoral threshold for parties is applied.
Two-vote MMP and additional member system systems may also be interpreted as a related, effectively preferential mixed system. Votes are not transferred, but a voter may vote differently for the local election and the overall party vote, with one, both or neither of those votes electing someone.
Sequential proportional approval voting is similar to STV by awarding seats to winners in a round-by-round basis and resulting in proportional representation but contrasts to STV by utilizing approval voting and re-weighting ballots between rounds instead of ranking candidates and transferring votes.[10]
District magnitudes and proportionality[edit]
Formally, STV satisfies a fairness criterion known as proportionality for solid coalitions.[29]
Historically, the district magnitude under STV elections has ranged from two (the absolute minimum) to 21 (currently being used in New South Wales, Australia to elect the Legislative Council, half at a time). In higher-level government elections district magnitude is usually in the 3 to 5 or 7 range, with NSW being an obvious exception.[30][31] In local government elections such as city councils, STV elections are often held citywide with district magnitudes in the 6 to 13 range, or wards may be used, usually electing 2 to 5 members in each ward.[32][33][34][35]
If the Droop quota is used, for example, in a nine-seat district, the quota or threshold is 10% (plus one vote); in a three-seat district, it is 25% (plus one vote). This electoral threshold is significantly higher than for most party-list PR systems, but a set fraction of the votes cast in a district covering just part of a jurisdiction may involve an equivalent or fewer number of votes as compared to a smaller percentage across a whole jurisdiction.
District elections grow more proportionally representative in direct relation to the increase in the number of seats to be elected in a constituency – the more seats, the more the distribution of the seats in a district will be proportional. For example, in a three-seat STV election using the Hare quota of , a candidate or party with at least one-third of the votes is guaranteed to win a seat. In a seven-seat STV contest using the Hare quota, any candidate with one-seventh of the vote (either first preferences alone, or a combination of first preferences and lower-ranked preferences transferred from other candidates) will win a seat. Many systems use the Droop quota, which is even smaller than the Hare for the same number of seats, as it produces more proportional results.
Because of this quota-based fairness, under STV it is extremely rare for a party to take a majority of the seats in a district without the support of a majority of the district's voters. Additionally, a large majority of voters (generally around 80 percent or more) see their vote used to elect someone. Thus under STV, the candidates who make up a majority of the district's elected members are supported directly by a majority of the voters in the district.
Benefits[edit]
Advocates for STV argue it is an improvement over winner-take-all non-proportional voting systems such as first-past-the-post, where vote splits commonly result in a majority of voters electing no one and the successful candidate having support from just a minority of the district voters. STV prevents in most cases one party taking all the seats and in its thinning out of the candidates in the field prevents the election of an extreme candidate or party if it does not have enough overall general appeal.
STV is the system of choice of the Proportional Representation Society of Australia (which calls it quota-preferential proportional representation),[82] the Electoral Reform Society in the United Kingdom[83] and FairVote in the United States (which refers to STV as proportional ranked choice voting[84] and instant-runoff voting as "ranked choice voting",[85] although there are other preferential voting methods that use ranked-choice ballots).