The critical concept of an election by STV to a party list of candidates is that elections for an ordered list are elections for a declining series of committees, starting with the whole list. So for a list of ten candidates, the first step is to determine who is on the list at all by conducting a count for ten places (subject perhaps to gender rules described below). No candidate who fails to get on the list at this point can get on the list at all. Then a count is conducted for nine places, with the one candidate who drops out being placed tenth. If two candidates drop out, as could happen, this is a tie between them, determined as usual by comparing their first preference votes. This process is continued iteratively until eventually a run-off between the top two determines the top of the list.
Since 1988 the Liberal Democrats have implemented rules for minimum proportions of women and men elected to Federal party committees. The principal objective of STV is the representation of opinions in proportion to their level of support amongst the electorate. In order to ensure that this is as far as possible maintained while implementing a gender balance rule, new developments in STV counting were put in place at that time and have been successfully used ever since.
The rule is expressed (a little inelegantly) in the party's election regulations as:
"If the Specified Proportions of men and women are not elected by the operation of the [standard STV] rules, the Returning Officer shall conduct such further counts as appear to the him or her to be necessary and declare elected those members of the under-represented sex and declare not elected those members of the over-represented sex who would or would not have been elected to committees with such larger and smaller numbers of members as would cause the correct number of that sex to be elected."
In practice this means that a stepwise procedure is followed.
The procedure does not allow anyone of the under-represented gender to be displaced, nor can anyone of the over-represented gender who was not elected in the first count gain election as a result of the additional counts. It is possible that the series of counts will not elect precisely the correct number of one gender. For example in one election counts for different sized committees elected 3 or 5 but not the required 4 women. This was treated as a tie between the two women not in the set of 3, determined by their numbers of first preference votes in the original count.
Selecting an ordered Party List page