22 September 2010

Your mission... to rank or not to rank?

Help!
A conversation on a (hijacked) Facebook thread has thrown up a conundrum: to rank or not to rank candidates you don't like in STV elections.
There are differing views.I recall a presentation from an electoral officer, demonstrating the iterations of STV computations, that suggested it was better not to rank your enemy, rather than ranking them lowly.  If you rank someone, you risk transferring some of your single vote to them, hence helping them get elected.
Others, including our resident electoral boffin, Graeme Edgeler, think this is daft and disagree.  They've tried to explain in narrative form why I must be wrong.  But I'm not yet convinced.
Your mission, if you choose to accept it, is to demonstrate who is right.  And best that the proof shows some example iterations or other evidence that proves or disproves one of the views.  We're not going to resolve this debate merely through a voice vote!
This message will self-destruct in 5 seconds.  Well, perhaps once we find an answer...

4 comments:

Phil Lyth said...

Speaking as an STV geek, the way I heard it expressed years ago (by a dinosaur) was

"No-one can game STV. A higher choice cannot override a lower preference"

Rob Carr said...

Hi Dean, I am not going to do a sample but it is relatively easy to answer the question. The most important thing is to rank every other candidate that way you have maximum chance someone other than your disliked person will win.

If there is only 1 disliked candidate it is irrelevant whether you rank because they cannot get your last preference if you rank all candidates. The iterations stop when only one candidate is left.

If there are multiple disliked candidates and you dislike them equally you should rank all other candidates first then not rank them because it is pointless.

If there are multiple disliked candidates and you dislike them to differing degrees you should rank them accordingly so at least you get the lesser of two evils.

Essentially whether you rank them or not is not important so long as you rank every other candidate.

Graeme Edgeler said...

Dean - you do risk some of your vote being transferred to them, and helping them be elected, but only at the expense of those you want to be elected even less.

But because my re-stating my point repeatedly won't help solve our conundrum, I've sent off an email to the STV people at Internal Affairs. Some time in the next 20 working days, but probably sooner, we should have the official position.

Phil - tactical voting is possible in all voting systems with three or more candidates.

ref: http://en.wikipedia.org/wiki/Gibbard-Satterthwaite_theorem

Graeme Edgeler said...

I've been think about it more and am convinced I am right (this actually doesn't happen all that often).

Look at the results of the 2007 WCC elections: by the time Kerry was elected mayor on the ninth iteration, the votes of more than 8000 people had stopped counting, because they hadn't numbered all the way to the end. If those people had gone to the end, and voted Kerry last, she wouldn't have been elected on the ninth iteration, and probably wouldn't have been elected at all.

Voting someone as person 20 out of 21 cannot harm the prospects of anyone you numbered 1 ~ 19. If there is someone you really don't want, vote them last.

Course Outline

Lord Justice Lawton in Maxwell v Department of Trade and Industry [1974] 2 All ER 122 said:

"From time to time ... lawyers and judges have tried to define what constitutes fairness. Like defining an elephant, it is not easy to do, although fairness in practice has the elephantine quality of being easy to recognise. As a result of these efforts a word in common usage has acquired the trappings of legalism: 'acting fairly' has become 'acting in accordance with the rules of natural justice', and on occasion has been dressed up with Latin tags. This phrase in my opinion serves no useful purpose and in recent years it has encouraged lawyers to try to put those who hold inquiries into legal straitjackets.... For the purposes of my judgment I intend to ask myself this simple question: did the [decision-maker] act fairly towards the plaintiff?"


This course examines the elephantine concept of fairness in the law, along with other contemporary legal issues.

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