[EM] PR-STV and vote management

[raphfrk at netscape.net]

 James Gilmour jgilmour at globalnet.co.uk 
 
 > > raphfrk at netscape.net > Sent: 20 April 2007 16:51
 > >  Under the standard rules,

> > you are not making any claim about what candidates you want to get

> > elected, you are just ranking them.

> 

> I don't know what "standard rules" you have in mind, but that does seem

> to me to be a perverse interpretation of marking preferences in an

> STV-PR election.  If there are 3 places to be filled and you have marked

> preferences for 5 of the 9 candidates, you are surely saying that "the

> candidates you want to get elected" are those three you have marked "1",

> "2" and "3".  You may not achieve that (because not enough other voters

> agree with you), but that is surely what you are claiming you want.





Optimal strategy in PR-STV is to rank all the candidates (actually all bar 1).  

What you are saying is the order in which you would like to elect a candidate.

You shouldn't stop ranking once you hit a candidate that you hate (unless you

hate all the remaining candidates equally).  The only time it matters is when

all your more preferred candidates have been elected or eliminated and the

effect of your vote will be to help elect a candidate (or if you prefer

your vote will be used to help eliminate a candidate you don't like).



If the choices were great, good, bad and Hitler, you should vote



great: 1

good: 2

bad: 3



Your vote for bad doesn't mean that you would like him to get elected,

it just means that you prefer him to Hitler.  Your vote will only go to

bad if the other 2 are eliminated or elected and he thus he may need it 

to beat Hitler (which is what you want to happen in that situation).



Under the system where candidates have to get the quota no matter what, 

then the situation changes.  The new best strategy is to rank all the

candidates except for candidates who you prefer less to the seat being

left vacant.



In the above example, you might vote:



great: 1

good: 2



This means that your vote will help one of the 2 to get elected, but

will never help bad or Hitler get elected.  It could deadlock the

election and leave the seat vacant.

 
 
  Raphfrk
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