Single Transferable Vote PR
Unlike party-list PR, Single Transferable Vote PR (STV PR) does not rely on parties. While STV can be (and is) used in partisan elections, it is also suitable for non-partisan elections, and is the only fully proportional system in use for that purpose. This makes STV especially useful for non-partisan elections such as those for city councils and school boards.
While Single Transferable Vote (STV) comprises a family of variations, the underlying method is the same:
- Ranked ballot. Each voter ranks some or all of the candidates in order of preference.
- Establish quota. The quota is the threshold number of votes required to elect a candidate.
- Elect winners. The first choices of all the ballots are tallied, and the candidates receiving at least a quota of votes are elected.
- Finished? If the number of candidates remaining is equal to or less than the number of unfilled seats, the remaining candidates are elected. If all seats are filled, or no candidates remain, the count is complete.
- Transfer surplus. If seats remain to be filled, transfer votes received by elected candidates in excess of the quota to the next choice on the ballots, and resume the process with Elect winners.
- Defeat low candidate. After all surpluses have been transferred, if seats remain to be filled, defeat (eliminate) the candidate with the fewest first-place votes, and transfer that candidate’s ballots to the next candidate listed on each of those ballots. Continue at Elect winners.
Eventually, either all seats will be filled or all candidates will be elected.
Each voter has a single vote (hence the name “single transferable vote”), which is eventually counted toward one of the candidates (or, in some STV methods, divided among multiple candidates). All or part of a vote can be non-transferable and not counted if the ballot, for example, ranks only candidates who have been defeated.
Evolution of STV methods
Specific STV methods vary in how arithmetic is performed (precision and rounding), how the quota is calculated, and how ties are treated (ties can arise in Elect winners and Defeat low candidate). Most important, STV methods differ in the manner in which they transfer surplus votes from elected candidates to still-hopeful candidates. Elsewhere, we discuss the evolution STV variations in a historical context, looking at how increasingly sophisticated methods have been developed. This evolution was a consequence of a better understanding of the mathematics of election methods, along with the availability of calculators and computers to aid vote counting.
STV in a Nutshell
Hoag & Hallett (1926) offered perhaps the simplest-ever set of instructions for an election that gets at the basic idea of STV. They ask us to imagine the election of a five-member council, carried out in a park. Nine candidates have been nominated. Here are the rules:
- To vote, stand beside the banner of the candidate who you prefer.
- Change your vote as many times as you please until the polls close.
- When the polls close, the five candidate who are surrounded by the five largest groups of voters will be declared elected.
The basis of STV lies in the recognition that voters who stand by a candidate with very few votes are motivated to move and help elect their second choice, and that a candidate with more than 1/6 of the votes can’t lose, so any votes beyond that are also free to help elect their second choice.
For more complete, but only slightly more complicated, description of the principles behind STV, see STV in a Nutshell.
Given that proportional representation is desired, why would we choose the STV form of PR?
When proportional representation is a requirement for a non-partisan election, STV PR is an easy choice over list PR, because there are typically no entities (most often parties) to create the candidate lists required for list PR. We see this in Cambridge MA and Minneapolis MN, where STV PR is used for non-partisan municipal elections.
STV PR can also be used for partisan elections. While we don’t have an example of partisan STV elections in the US, the Irish lower house of Parliament, Dáil Éireann, has been elected with STV since 1922.
When proportional representation is required, we can dispense with such majority-take-all systems such as plurality-at-large, and approval voting, or single-member districts with any voting method.
We do, however, sometimes see semi-proportional voting methods, most often cumulative voting or single non-transferable vote (SNTV), proposed. It’s useful to understand why such methods are much less satisfactory than STV PR for delivering proportional results.
We apply the term “semi-proportional” to voting methods in which a minority can achieve some measure of representation through voting power, but where full proportionality is not guaranteed. The lack of full proportionality can be due to the absence of fully proportional voting power, or, more likely, an implicit requirement that a voting bloc perfectly coordinate their votes in order to achieve full proportionality.
See Semi-proportional Electoral Methods for a discussion of the problems of those methods.