During next May's provincial election, BC voters will be asked, for the second time, whether or not they support the introduction of the Single Transferable Vote (STV). At the moment, we simply cast a vote for our favoured candidate and the candidate collecting the most votes wins. Under the STV, we'll be asked to rank candidates in descending order of preference and the winner would be selected by a complicated counting formula.
One precedent for a preferential system is the City of Vancouver. In 1920 civic voters approved a change to the city's electoral system and in the next election, early in 1921, voters marked their choices for mayor and aldermen in order of preference, 1, 2, 3, and so on. When ballots were counted, the candidate receiving the fewest votes was eliminated and his or her second preferences were distributed among the remaining candidates. This went on until one candidate received at least 50 percent of the vote and was declared the winner.
The new system was not a success. In the days before computers, electoral officials hated having to count and recount ballots. More importantly, the preferential ballot did not have any impact on the outcome. In the overwhelming majority of cases, the candidate who led on the first count ended up winning in the end.
After three elections the city decided there was no point continuing with the more complicated system and reverted to the single-choice ballot
Of course, the preferential ballot was a far cry from the proposed province-wide STV. Still, it is a cautionary tale. Watch out for unintended consequences. Or, in the case of Vancouver in the 1920s, no consequences at all.