III. Basic Structure of STV

The basic structure of an STV method can be described as follows:

1. Establish the winning threshold.
2. Count the first place votes.
3. If one or more candidates have surplus votes (votes in excess of the winning threshold), then transfer votes from the candidate with the largest surplus. Otherwise, eliminate the last place candidate and transfer those votes. The only candidates who can receive votes are those who are under the winning threshold and not eliminated. If there are no further candidates on a ballot than that vote is "exhausted" and does not count towards any candidate.
4. If there are more than 4 candidates remaining, then go to step (3).

The above is ambiguous in that it does not define the winning threshold and does not describe precisely how votes are to be transferred. Specific implementations of STV define both of these and may also modify the basic structure.

The winning threshold is also called a quota. I avoid the term quota because it has negative connotation in the US. A commonly used threshold is defined as

                number of votes
threshold =  --------------------- + 1
              number of seats + 1

and then dropping any fraction.

This is the smallest threshold such that any candidate receiving this number of votes is guaranteed to be elected. This will be discussed in more detail below.

Methods for transferring surplus votes can be classified into two main groups: random transfer of votes and fractional transfer of votes. Methods for transferring votes from eliminated candidates do not vary greatly.