Strategy to *not* vote for a candidate using Single Transferable Vote

Question

If I was to participate in a election using the Single Transferable Vote system, what would be my best strategy to try and prevent one candidate from winning.

For example if there are 5 candidates. One who I would like to win, one I very much would not like to win (Plus 3 who I don't have much opinion either way).

It is however most important to me that the one I dislike does not get in (I disagree strongly with their policies etc.)

What would be my best strategy?

• Vote for all with Mr/Ms bad policy in last place?
• Vote for all Except Mr/Ms bad policy?
• Vote for just my top 1,2, or 3?
• Something else?

Is there a difference if there is only one seat, or several seats?

Answer 1

Giving your last preference for a candidate and not voting for them at all are equivalent in all forms of STV that elect a fixed number of people.

Your first two options are therefore equivalent.

Leaving out more than one person runs the possibility that one of them might be excluded rather than the one candidate you strongly dislike when your vote could have reversed that.

If you're determined to vote tactically, you should vote for all candidates other than the one you want to lose in reverse order of their popularity, ie from least popular (with other voters) to most. This assumes that you have some knowledge of how other voters will vote. This strategy is designed to minimize the chance that your vote ends up in a surplus, and therefore minimizes the risk of your vote being reduced in value by being surplus-transferred.

[Footnote, you can use STV with a fixed quota rather than a fixed number of winners; this allows for constituencies to automatically adjust depending on turnout, but also means you don't know in advance how many winners there will be, as high turnout will increase parliament size. In that case, there is a difference between a full ballot and an all-but-one ballot; you should only include candidates that are better than no representative. I'm not aware of any election in which this system is actually used.]

Answer 2

The standard form of Single Transferable Vote goes:

• A candidate who has reached or exceeded the quota is declared elected.
• If a candidate has more votes than the quota, surplus votes are transferred to other candidates. Votes that would have gone to the winner go to the next preference.
• If no-one new meets the quota, the candidate with the fewest votes is eliminated and those votes are transferred.
• This process repeats until either a winner is found for every seat or there are as many seats as remaining candidates.

While (as @DVK linked) there's no general strategy to elect someone, the bolded part is key to avoiding someone getting elected.

Since the remaining candidate with the fewest votes is eliminated first, you want everyone else to have more votes than the one you wish to be eliminated. That means casting a vote for everyone except the one you dislike.

Answer 3

I've read through the article you've posted, and I think that you probably want to rank your vote in the order of likeliness to win, and simply not vote for the candidate that you don't want to get elected.(or if you have to rank him, rank him last.

Not only does this strategy maximize the chance of the person who you don't want to win to get eliminated, but It also maximizes the chance that other people get elected.

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If you are correct in your analysis of who is the most likely to win, than your vote will always be helping elect someone who is not that candidate, and can fill all the seats before that candidate wins.

Answer 4

There is no other sensible answer except to do what makes sense to do: rank your favorite candidate first and your least favorite candidate last. Anything else is nonsense.