Single Transferable Vote

The Single Transferable Vote, or STV, is a preferential voting system designed to minimise wasted votes and provide proportional representation in multi-candidate elections while ensuring that votes are explicitly for candidates rather than party lists. STV systems achieve this by initially allocating an individual's vote to their most preferred candidate and then subsequently transferring unneeded or unused votes after candidates are either elected or eliminated according to the voter's stated preferences.

Issues

A frequent concern with STV among electorates considering its adoption is its relative complexity compared with plurality voting methods. For example, when the Canadian province of British Columbia held a referendum on adopting the Single Transferable Vote in 2005, according to polls a majority of "no" voters gave their reason as "wasn't knowledgeable" when they were asked why, specifically, they voted against STV.{{ref|BC-STV_Video}}

Related Topics:
British Columbia - 2005

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However, as with all voting systems, once STV is understood there remain a number of areas of controversy surrounding its use. In particular, arguments for and against proportional representation in general are frequently referenced in debates among electorates considering STV, however the specific implications of a particular STV system can be examined as well.

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Proportionality

The outcome of voting under STV is proportional within a single election to the collective preference of voters, assuming voters have ranked their real preferences. However, due to other voting mechanisms usually used in conjunction with STV, such as a district or constituency system, an election using STV may not guarantee proportionality across all districts put together. Differential turnout across districts, for example, may alter the impact of individual votes in different constituencies, and rounding error associated with a finite number of winners in each constituency may throw up anomalous results. The New South Wales Legislative Council, where the whole state votes as a single electorate for 21 members, produces results that are proportional to the final allocation of preferences. In STV elections to the Australian Senate, states with vastly different populations have the same number of seats, and so while the results for individual states are proportional, the nationwide result is not, giving greater voting power to individual voters in less populated states; however, this lack of proportionality is derived from malapportionment rather than any deficiency in STV.

Related Topics:
New South Wales Legislative Council - Australian Senate - Malapportionment

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STV provides proportionality by transferring votes to minimise waste, and therefore also minimises the number of unrepresented voters. In this way STV provides Droop proportionality - an example STV election using the Droop quota method for 9 seats and with no exhausted preferences would guarantee representation to every distinct group of 10% of the voters, with at most only 10% of the vote being wasted as unneeded excess. Unlike other proportional representation methods employing party lists, voters in STV do not explicitly state their preferred political party. One common method of estimating the party identification of voters is to assume their top-preference on their ballot represents a candidate from their preferred party. This method is made more complicated by the possibility of independent candidates and of cross-party voting.

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Nevertheless, failures to produce partisan proportionality exactly analogous to the party affiliations of top choice candidates as occurs in list PR elections can be controversial. For example, in Malta in 1981 the party winning more than half the votes won less than half the seats, resulting in a constitutional crisis: see below. Similarly the Northern Ireland elections in 1998 led to the Ulster Unionists winning more seats than the Social Democratic and Labour Party, despite winning a smaller share both of first-preference votes and of votes after transfers.

Related Topics:
List PR - Malta - 1981 - Constitutional crisis - Below - Northern Ireland elections - 1998 - Ulster Unionist - Social Democratic and Labour Party

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Tactical voting

The single transferable vote eliminates much of the reason for tactical voting. Voters are "safe" ranking candidates they fear may not be elected, because their votes will be transferred after they are eliminated. Similarly, voters are also "safe" voting for a candidate they believe will receive overwhelming support, because their votes will then get reallocated to their next preference, though with less than the value of a full vote. However, the Gibbard-Satterthwaite theorem states that tactical voting is possible in any deterministic voting system where any candidate can win, and that STV is no exception.

Related Topics:
Tactical voting - Gibbard-Satterthwaite theorem - Deterministic

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Tactical voting is chiefly accomplished in STV by making assumptions about the other voters. A preferred popular candidate can be assumed to win and thus ranked lower on a tactical voter's ballot, allowing the voter to give more weight in transfers to his second-choice candidates (and, implicitly, giving fellow supporters of the popular candidate less weight.) This is particularly effective in the older STV systems still used in many countries that prevent elected candidates from receiving additional votes, as in that case none of the tactical vote is diluted on the already winning candidate. Under such old systems, a voter may even rank a non-preferred candidate that is assumed to lose first in order to increase the chances of his vote arriving late. This method of tactical voting is much less effective in the New Zealand STV system using Meek's method, however, as votes receive the same fractional weighting regardless of when they arrive at the successful candidate.

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Though still theoretically possible, figuring out how to successfully vote tactically in modern STV systems by exploiting the non-monotonicity in this way can be computationally difficult. It is NP-hard to determine whether there exists an insincere ballot preference that will elect a preferred candidate, even in an election for a single seat.{{ref|Bartholdi}} This makes tactical voting in STV elections vastly more difficult than with other commonly-used election methods. Importantly, this resistance to manipulation is inherent to STV and does not depend on hopeful extraneous assumptions, such as the presumed difficulty of learning the preferences of other voters. Furthermore, it is also NP-hard to determine when an STV election has violated the monotonicity criterion, greatly reducing the likelihood that the electorate will know if even accidental tactical voting has occurred. As a consequence, the difficulty of tactical voting in STV elections increases sharply as the number of voters, candidates, and winners increase. This gives an incentive for larger electoral districts other than their increased proportionality, since particularly small electoral districts may have few enough candidates to make tactical voting feasible.

Related Topics:
Non-monotonicity - NP-hard

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Of special note is that voters have a real incentive to list their preferences honestly in STV, as it is the best strategy for securing representation if tactical voting is either impractical or impossible. This is frequently the case in large electoral districts, as successful tactical voting (when possible) requires both nearly perfect information about how others are voting and the computation of a virtually unsolvable math problem. Since tactical voting in STV works by effectively substituting one's own alternate preferences for transfers with other supporters of the same candidate, the effectiveness of tactical voting is greatly reduced when other supporters of preferred candidates have similar second-choice preferences. Although there is no way to completely prevent tactical voting by hiding support for preferred candidates, the tactical voter carries the significant danger of his assumptions about the popularity of his preferred candidate being wrong, risking his most preferred candidate losing because of his miscast tactical vote. This contrasts heavily with non-proportional, plurality-based systems, where there is both tremendous incentive and ability to vote tactically in order to avoid the spoiler effect.

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Effects on factions and candidates

The use of STV may reduce the role of political parties in the electoral process and corresponding partisanship in the resulting government. Unlike proportional representation systems employing party lists, voters in STV are not explicitly constrained by parties even when they do exist; voters may ignore candidate party labels and mix their preferred candidate rankings between parties. Similarly, candidates may achieve electoral success by obtaining a quota of voters not generally within their own party, perhaps by winning transfers from moderates or by championing a specific issue contrary to party doctrine. Unlike List PR, STV can be used in elections in organizations without any political parties at all, such as in trade unions, clubs, and schools.

Related Topics:
Partisanship - Party list - Trade union - Club - School

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Some STV variations, however, may encourage the role of political parties and actually strengthen them. In Australian Senate elections, where a combination of large districts, mandatory complete ballots, and compulsory voting results in the near 95% usage of partisan group voting tickets, political parties gain significant power in determining election results by adjusting the relative ordering of their tickets.

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Successful campaign strategy in STV elections may differ significantly from other voting systems. In particular, individual candidates in STV have little incentive for negative campaign advertising, as reducing a particular opponent's ranking among voters does not necessarily elevate one's own; if negative campaigning is seen as distasteful by the voters, the practice may even prove harmful to the attacking candidate. Conversely, in order to avoid elimination in early counting rounds by having too few first place votes, candidates have a significant incentive to convince voters to rank them explicitly first as their top preference, rather than merely higher. This incentive to attain top preferences, in turn, may lead to a strategy of candidates placing greater importance on a core group of supporters. Avoiding early elimination, however, is usually not enough to win election, as a candidate must still subsequently win enough votes on transfers to meet the quota; consequently, strategies which sacrifice wide secondary support in favor of primary support amidst a core group may ultimately fail unless the group is particularly large.

Related Topics:
Campaign - Campaign advertising

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There are also tactical considerations for political parties in the number of candidates they stand in an election where full ballots are not required. Standing too few candidates may result in all of them being elected in the early stages, and votes being transferred to candidates of other parties. Standing too many candidates might result in first-preference votes being spread too thinly amongst them, and consequently several potential winners with broad second-preference appeal may be eliminated before others are elected and their second-preference votes distributed. This effect is amplified when voters do not stick tightly to their preferred party's candidates; however, if voters vote for all candidates from a particular party before any other candidates and before stopping expressing preferences, then too many candidates is not an issue. In Malta, where voters tend to stick tightly to party preferences, parties frequently stand more candidates than there are seats to be elected. Similarly, in Australian Senate elections, voters also tend to vote along party lines due to the relative ease of selecting a party's declared preferences rather than individually casting their own complete list.

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Voting system criteria

Academic analysis of voting systems such as STV generally centers on the voting system criteria that they pass. No system satisfies all the criteria described in Arrow's impossibility theorem: in particular, STV fails to achieve independence of irrelevant alternatives (like most other vote-based ordering systems) as well as monotonicity. Failure to satisfy independence of irrelevant alternatives makes STV slightly prone to strategic nomination, albeit less so than with plurality methods where the spoiler effect is more pronounced and predictable. Non-monotonicity, in turn, makes it possible under some circumstances to elect a preferred candidate by reducing his position on some of the ballots; by helping elect a candidate who displaces the preferred candidate's main rival, a voter may cause the preferred candidate to profit from transfers resulting from the rival's defeat. STV fails the participation criterion which can result in a more favorable outcome to an STV voter by not voting at all. However, STV satisfies the Non-compulsory support criterion. A voter who truncates a candidate off the ballot does not harm a ranked candidate, nor is another truncated candidate helped on the ballot.

Related Topics:
Voting system criteria - Arrow's impossibility theorem - Independence of irrelevant alternatives - Monotonicity - Strategic nomination - Spoiler effect - Participation criterion - Non-compulsory support criterion

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STV is also susceptible to the Alabama paradox: if a candidate is elected in an n seat constituency, she may not be elected in the same constituency with n + 1 seats even when voters express exactly the same preferences. This is due to the use of quotas; list PR by a largest remainder method is similarly affected, though a highest averages method is not. Intuitively, a candidate who was elected largely because of transfers from two similar groups (neither obtaining a quota) may not be elected when the number of winning candidates increases, as both groups would instead get their preferred candidates elected (with the new, smaller quota) rather than automatically compromising on their mutual second choice as their votes transfer.

Related Topics:
Alabama paradox - Largest remainder method - Highest averages method

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Some modifications to STV have been proposed in order to pass monotonicity and other criteria. The most common method of proposed modification to STV is to alter the order in which candidates are eliminated: theoretically, a candidate who ranked second on every ballot could be the first candidate eliminated even if he is a Condorcet winner. Meek noted this problem in proposing his variation of transferring votes to nearly eliminate tactical voting in STV, however Meek himself did not propose a method for satisfying the Condorcet criterion. Other theorists have proposed further refinements of STV, such as using a Condorcet method to rank candidates for elimination order. Some of these modifications alter STV in a way such that it no longer reduces to instant-runoff voting when applied to a single seat but instead reduces to some other single winner system, such as a Condorcet method.

Related Topics:
Condorcet winner - Condorcet criterion - Condorcet method - Single winner system

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See also: CPO-STV, Quota Borda system

Related Topics:
CPO-STV - Quota Borda system

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District size

Another issue commonly considered with STV elections is the size of the voting districts in terms of the number of candidates elected and, to a lesser extent, the total size of the body being elected. In STV and other proportional representation systems, a larger number of candidates elected results in a smaller number of wasted votes and subsequent rounding error, leading to a more proportionally representative outcome based on the preferences of the voters. For this reason, larger electoral districts can also significantly reduce the effects of gerrymandering; because gerrymandering relies on wasted votes to award the "last seat" in each district, proportional representation systems such as STV with larger multimember districts are intrinsically more difficult to gerrymander.{{ref|Whyte}} Larger districts can also make for significantly harder tactical voting: since the problem of making correct assumptions about other voter's behavior and rearranging one's tactical ballot is NP-hard, the difficulty of tactical voting increases sharply as the number of candidates grows. Some STV elections make use of districts with the number of seats available as small as three, however there is no theoretical upper limit to the size of districts in STV, and they may not even be needed at all: Thomas Hare's original proposal was for a single, nationwide district.

Related Topics:
District - Gerrymandering - NP-hard

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However, because STV is proportional, larger elections also reduce the support a candidate requires to become elected as a percentage of the district. With 9 candidates, for example, any with 10% electoral support may win a seat, whereas with 19 candidates only 5% popular support is required to be in the government. Because of this, proportional representation methods such as STV can implicitly allow for the election of particularly small minorities provided they secure a quota's worth of votes. The potential admission of small voting minorities into the government, perhaps including political extremists, can be quite controversial even though they would occupy only an equivalently small proportion of the government once elected. Other proportional representation methods attempt to avoid the election of small political minorities by imposing partisan threshold requirements, such as the "5% rule" in Germany's Mixed Member Proportional system. While impossible at the national level, such a threshold requirement could be approximated in STV on a per-district basis by limiting the number of seats to 19 (and therefore making the quota one more than 5% of the vote). Larger districts and the implicit larger number of candidates also increase the difficulty of giving meaningful rankings to all candidates from the perspective of the individual voter, and may result in increased numbers of exhausted ballots and reliance on party labels or group voting tickets.

Related Topics:
Extremist - Germany - Mixed Member Proportional

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Vacancies

When compared with other voting methods, the question of how to fill vacancies which occur under STV can be difficult given the way that results depend upon transfers from multiple candidates. There are several possible ways of selecting a replacement:

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Countback

The countback method is used in the Australian Capital Territory, Tasmania, Malta, and Cambridge, Massachusetts. A new representative is selected by using the data from the previous election. The candidate who held the seat is eliminated, and a new election result is obtained by transferring votes from the now unrepresented voters. Importantly, a clone of the replaced representative would be guaranteed to win the vacated seat. An interesting consequence of the countback method's use of existing ballots for selecting replacements is that the results are often known before the vacancy actually occurs, potentially influencing the circumstances which create the vacancy in the first place.

Related Topics:
Australian Capital Territory - Tasmania - Malta - Cambridge, Massachusetts - Clone

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Although the countback method is designed to select a replacement representing the same group of voters who elected the original candidate, it remains possible that no similar candidates remain on the ballot. In 1985 the Tasmanian parliament amended the electoral act to allow true by-elections if no candidates of the same party as the outgoing representative remained on the ballot; in this circumstance the party may request that a by-election be held, however this has not yet happened.

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Countback methods vary by whether or not wasted and exhausted ballots are additionally used during the countback. The effect this has on the result of the countback depends on the differences in the next preferences of voters; if there exists a true clone in the ballots, there should be no effect in this change. Moreover, in STV systems that use exhausted ballots during countbacks, it becomes theoretically possible that the order of multiple resignations will affect who the ultimate replacements are - this is a consequence of the non-independence of irrelevant alternatives discussed above. Additionally, if ballots are allowed to be exhausted in the election, then by either method it remains possible that the chosen replacement will only meet a fraction of a quota of voters; when this fraction is particularly small, and therefore no similar candidates remain on the ballot, election rules may call for a different method of filling the vacancy to be used.

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Appointment

Another option is to have a head official or remaining members of the elected body appoint a new member to fulfil the vacancy. In Australia, for example, the state legislatures appoint replacements members to the Australian Senate, now done at the suggestion of the party of the outgoing senator. Before this rule, disputes over Senate vacancies contributed to the Australian constitutional crisis of 1975, ultimately resulting in a 1977 amendment to the Constitution of Australia to provide that the legislature must elect a member of the same party as the outgoing senator. Vacancies in the New South Wales Legislative Council are filled in a similar way by a joint sitting of both the legislative council and assembly.

Related Topics:
Australian constitutional crisis of 1975 - Constitution of Australia

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In the Republic of Ireland, vacancies on local authorities are filled by co-option of a candidate nominated by the departed councillor's party colleagues.{{ref|IrlLocAuthRep}} Non-party candidates are not replaced till the next scheduled election.

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By-election

A third alternative to fulfill a vacancy is to hold a single-winner by-election (effectively instant-runoff), as happens in the Dáil and Seanad in the Republic of Ireland; this allows the parties to choose new candidates and all voters to participate, but often leads to the most popular party picking up an extra seat. This is because the winner of a by-election typically represents a large group of voters, whereas the vacated member only represented a particular quota's worth.

Related Topics:
By-election - Instant-runoff - Dáil - Seanad

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Replacement List

Departed Members of the European Parliament are replaced with the top eligible name on a replacement list submitted by the candidate at the time of the original election. {{ref|IrlMEPRep}}

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