Single Transferable Vote (STV)

No, its not an STD. This is the system which the Liberal Democrats want to replace the first-past-the-post district by district UK system with.

And the thing is, it would likely lead to them controlling a lot closer to the 23% of Parliament they would control in a proportional election. (Which would in turn likely increase their popularity, as voting for them would no longer be a "throwaway" vote).

And the other thing is, it has some chance of happening, because the Lim Dems are (kind of) in the role of Kingmaker (kind of because they cant, quite, make Labour kings as the graphic from earlier today shows). The conservatives, in a an effort to win over the LDs, have talked of compromise and setting up a committee to study the issue.. *yawn*.. Labour says they want a referendum on a different voting system, fixed-term Parliaments, and reducing the voting age to 16. Yeah, because 16 and 17yr olds should really be out there voting... golly gee, I wonder what party they tend to support? Anyway, it looks like there might be a jolly old shakeup in the UK voting system.

The STV system is pretty damn interesting, and as someone who feels completely unrepresented by the current system (I am about as aligned with the Republicans or the Democrats as a mouse is with a cat or an owl) I kind of support it (though I think it would lead to less getting done... wait as second, thats a good thing. I support less getting done. The US govt. hit its peak about 200 years ago).

Single transferable vote

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The single transferable vote (STV) is a preferential voting system designed to minimize "wasted" votes, provide proportional representation, and ensure that votes are explicitly cast for individual candidates rather than party lists. STV achieves this by using multi-seat constituencies (voting districts) and by transferring votes that would otherwise be wasted on sure losers or winners to other eligible candidates. STV initially allocates an elector's vote to his or her most preferred candidate and then, after candidates have been either elected or eliminated, transfers surplus or unused votes according to the voters' stated preferences. The single-winner variant of STV is known as instant runoff voting and produces results similar to a two-round electoral system rather than proportional representation.

In Tasmania, Australia and the Australian Capital Territory, STV is known as Hare-Clark, in recognition of Thomas Hare, who initially developed the system and the Tasmanian Attorney General, Andrew Inglis Clark, who worked to have a modified version introduced. Hare-Clark has been subsequently modified to allow for improvements, such as rotating ballot papers (the Robson Rotation). STV is the system of choice of groups such as the Proportional Representation Society of Australia and the Electoral Reform Society in the United Kingdom. Its critics contend that some voters find STV difficult to understand.[1]

Contents

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[edit] Adoption

STV has had its widest adoption in the English-speaking world. As of 2010, STV is used for:

Ireland Parliamentary elections (since 1919)
European elections
Local government elections
Malta Parliamentary elections
European elections
Local government elections
United Kingdom Northern Ireland Regional assembly elections
European elections
Local government elections
Scotland Local government elections (since May 2007)
Australia Country-wide Senate elections (in the form of a group voting ticket)
Tasmania House of Assembly elections
Local government elections
Australian Capital Territory Legislative Assembly elections
New Zealand Some local government elections such as Dunedin and the capital city of Wellington
Local health board elections
United States City elections in Cambridge, Massachusetts
Certain city elections in Minneapolis, Minnesota (starting in 2009)

In British Columbia, Canada, STV was recommended for provincial elections by the BC Citizens' Assembly on Electoral Reform. In a 2005 provincial referendum, it received 57.69% support and passed in 77 of 79 electoral districts. It was not adopted however, because it fell short of the 60% threshold requirement the Liberal government had set. In a second referendum on May 12th 2009 STV was defeated, 60.91% to 39.09%

STV has also been used historically in several other jurisdictions. For a more complete list, see History and use of the Single Transferable Vote.

[edit] Terminology

When STV is used for single-winner elections, it is equivalent to the non-proportional instant-runoff voting (alternative vote) method. To differentiate them, STV used for multi-winner elections is sometimes called proportional representation through the single transferable vote, or PR-STV. The term STV usually refers to the multi-winner version, as it does in this article. In Australia STV is known as the Hare-Clark Proportional method, while in the United States it is sometimes called choice voting, preferential voting or preference voting (note that preferential voting can alternatively refer to a broader category of voting systems).

[edit] Voting

Preferential ballot.svg

In STV, each voter ranks the list of candidates in order of preference. In other words (under the most common ballot design), they place a '1' beside their most preferred candidate, a '2' beside their second most preferred, and so on. The ballot paper submitted by the voter therefore contains an ordinal list of candidates. In the ballot paper shown in the image on the right, the preferences of the voter are as follows:

  1. John Citizen
  2. Mary Hill
  3. Jane Doe

[edit] Counting the votes

[edit] Setting the quota

In an STV election, a candidate requires a certain minimum number of votes – the quota (or threshold) – to be elected. A number of different quotas can be used; the most common is the Droop quota, given by the formula:

\mbox{votes needed to win} = \left({{\rm \mbox{valid votes cast}} \over {\rm \mbox{seats to fill}}+1}\right) + 1

The Droop quota is an extension of requiring a 50% + 1 majority in single winner elections. For example, at most 3 people can have 25% + 1 in 3 winner elections, 9 can have 10% + 1 in 9 winner elections, and so on.

[edit] Finding the winners

An STV election proceeds according to the following steps:

  1. Any candidate who has reached or exceeded the quota is declared elected.
  2. If a candidate has more votes than the quota, that candidate's surplus votes are transferred to other candidates. Votes that would have gone to the winner instead go to the next preference listed on their ballot.
  3. If no one new meets the quota, the candidate with the fewest votes is eliminated and that candidate's votes are transferred.
  4. This process repeats until either a winner is found for every seat or there are as many seats as remaining candidates.

There are variations in applying these STV rules, such as in how to transfer surplus votes from winning candidates and whether to transfer votes to already elected candidates. When the number of votes to transfer from a losing candidate is too small to change the ordering of remaining candidates, more than one candidate can be eliminated simultaneously.

Because votes cast for losing candidates and excess votes cast for winning candidates are transferred to voters' next choice candidates, STV is said to minimize wasted votes.

[edit] An example

Suppose a food election is conducted to determine what to serve at a party. There are 5 candidates, 3 of which will be chosen. The candidates are: Oranges, Pears, Chocolate, Strawberries, and Sweets. The 20 guests at the party have the preferences marked on their ballots in the table below. In the following table only some of the second preferences and none of the lower preferences are shown because they happen to not be needed in the count (a different set of votes could be constructed where first, second and third preferences of some voters must be considered).

# of Guests x x x x x x x x x x
x x x x
x x x x x x
1st Preference Orange Pear Chocolate Chocolate Strawberry Candy
2nd Preference
Orange Strawberry Candy

First, the quota is calculated. Using the Droop quota, with 20 voters and 3 winners to be found, the number of votes required to be elected is:

\left({\mbox{20 votes cast} \over {\mbox{3 seats to fill}+1}}\right) +1 = \mbox{6 votes required}

When ballots are counted the election proceeds as follows:

Candidate: Orange Pear Chocolate Strawberry Candy
Round 1 x x x x x x x x x x
x x x x

x x x x
x x Round 1: Chocolate is declared elected, since Chocolate has more votes than the quota
Round 2 x x x x x x x x x x
x x
x x x x
x
x x x Round 2: Chocolate's surplus votes transfer proportionately to Strawberry and Sweets according to the Chocolate voters' second choice preferences. However, even with the transfer of this surplus no candidate has reached the quota. Therefore Pear, who has the fewest votes, is eliminated.
Round 3 x x x x
x x

x x x x
x x
x x x x
x
x x x Round 3: Pear's votes transfer to their second preference, Oranges, causing Orange to reach the quota and be elected. Orange meets the quota exactly, and therefore has no surplus to transfer.
Round 4 x x x x
x x

x x x x
x x
x x x x
x
x x x Round 4: Neither of the remaining candidates meets the quota, so Sweets are eliminated. Strawberry is the only remaining candidate and so wins the final seat.

1 comment:

  1. Reweighted Range Voting and Asset Voting are simpler than and superior to STV.
    http://ScoreVoting.net/RRV.html
    http://ScoreVoting.net/Asset.html
    http://ScoreVoting.net/PropRep.html

    Also, Reweighted Range Voting has the nice property that it's single-winner form is exceptionally good, simple, and resistant to tactical voting. Whereas IRV ("alternative vote"), the single winner form of STV, is rather poor and strategically degrades into something roughly equivalent to plurality voting ("FPTP").

    http://scorevoting.net/CFERlet.html

    ReplyDelete