Countback
The countback method is a way of filling casual vacancies in proportional voting systems. Casual vacancies are filled by re-examining the ballots from the previous election. The candidate who held the seat is eliminated, and the election is then re-run with this candidate removed. Unlike other methods of filling vacancies, this procedure maintains proportional representation, and eliminates the need for expensive and low-turnout special elections.
Uses
Countbacks are used in Malta, the Australian Capital Territory, Tasmania, Victoria, and Western Australia, and for some local councils in New South Wales.Problems
Caused by STV
Countries often attempt to use countbacks with instant-runoff voting or single transferable vote, which often causes major problems and complications, because STV does not pass local independence of irrelevant alternatives. This means the results can respond chaotically to the removal of a winning candidate. For example, a second-place finisher could become the last-place finisher under a full STV recount, meaning a politician might lose their seat because of a colleague's death or resignation. Most jurisdictions that use STV countbacks avoid this by "resuming" the STV process in the final stage, taking votes originally allocated to the elected candidate and transferring them to any unsuccessful candidates. However, the procedure for this can quickly become complex and highly arbitrary, with the outcome depending strongly on minor variants in counting rules. This procedure also inserts spoiler effects into the race, allowing the final result to depend on the order in which vacancies arise, and potentially excluding popular candidates from contention because they were eliminated in early rounds.For rules that do pass LIIA, countbacks are trivial, because excluding the original winner causes the runner-up to take their place. Such rules therefore permit low-cost countbacks and allow jurisdictions to fully eliminate by-elections.
A further problem is that the number of exhausted ballots in STV elections increases every time a candidate is removed from contention. It is therefore possible that the replacement will be elected with only a handful of votes, substantially less than a full quota. If this fraction is particularly small, and thus no similar candidates remain on the ballot, election rules may call for a different method of filling the vacancy to be used.