Single Transferable Voting is used to try to achieve candidates that broadly reflect the wishes of the voters in multiple candidate elections. Similar to IRV, each voter expresses a preference for each candidate (1 for their top candidate, 2 for their second, etc.).
With STV there is a quota that a candidate needs to reach to be elected. This quota is based on the number of positions that can be filled, and is calculated by taking the number of positions, adding 1, and dividing the number of valid votes by that number. For example if there are 3 positions to be filled and 100 voters the quota is 100 divided by 4, so 25 votes. The quota must be exceeded (even by a fraction of a vote). The reason behind this is if 3 candidates all get more than 25 votes then any remaining candidates can never get more than 25 votes so will be eliminated.
The main difference between STV and IRV is that once a candidate exceeds the quota, the number of votes above the quota is passed onto next preference candidates that have not yet been elected or eliminated. This means for example that if there is one very popular candidate, the people who voted for this candidate will have a percentage of their second preference votes used to elect the next candidate, and so on.
The number of votes a candidate has received over the quota is called the surplus. The voters with next preference votes are divided equally by the surplus, and this fraction of each vote will be passed onto their next preference. This means candidates can get fractional parts of a vote in later rounds of calculation.
If all surpluses have been transferred, lower placed candidates will be eliminated (similar to IRV) and their next preferences re-allocated.
STV allows the candidates to be elected to broadly reflect the opinions of the voters. However it is more complex, and is also very time consuming to achieve in elections that are run manually. But they are relatively simple for any computer to process the calculations involved, so if voting is done using Anonyvoter then the advantages of STV can easily be achieved.
There are different variants of STV to reflect exactly how the calculations are achieved. Anonyvoter currently supports variants based on ERS97, Scottish STV and Minneapolis STV. The main differences deal with whether surpluses are always distributed when first available or can be deferred, and whether to always transfer the full surplus of votes or only those that caused the candidate to exceed the quota.
Anonyvoter bases its calculations on these variants, but there may be minor differences in rounding and tie-breaking. We are working to standardise these.
is a manual method of calculating STV calculations, which we have adapted for Anonyvoter.
The main variations are that surpluses will be deferred to a later calculation round if they are small enough that they won't affect who is to be eliminated first, and from the second calculation round onwards only the votes that were transferred to allow a candidate to exceed the quota will be then transferred to next preference candidates.
is used in Scottish Local Government elections. It's main differences to ERS97 are that surpluses are always transferred where possible (even if they make no difference to who will be eliminated first), and all votes are used to consider next preferences for candidates that exceed the quota, rather than just those last transferred that helped a candidate exceed the quota.
Minneapolis STV (Ranked Choice Voting)
is a cross between ERS97 and Scottish STV. Surpluses can be deferred (similar to ERS97), but all votes are used to consider next preferences for candidates that exceed the quota (similar to Scottish STV), rather than just those last transferred that helped a candidate exceed the quota.
Which version to choose?
In 99% of real-world cases it does not really matter. We have found that the elected candidates will change depending on which STV variant is used in less than 1% of all STV polls that have been run on Anonyvoter. The first rounds of calculation are the same, and usually there are only small variations in the votes that are transferred for successful and eliminated candidates. Occasionally different vote transfer rules may mean some of the borderline candidates in close elections may be elected in one variant and not another, but this is quite rare.
We have picked ERS97 as a default, but you can choose any of the other options we offer if you or your organisation have another preference.