[EM] How to make a summable version of STV

[Jameson Quinn]

2011/7/23 Kristofer Munsterhjelm <km_elmet at lavabit.com>

> Jameson Quinn wrote:
>
>> This method leaves open the possibility for "candidate hijacking".
>> Consider: Sojourner Truth has 1000 votes who vote Truth > Douglass >
>> Everyone. Robert E. Lee has 1100 strategic voters who vote Truth > Lee >
>> Everyone > Douglass. They dominate the honest Truth voters, so Lee is
>> considered to be Truth's second choice. The Droop quota is 1900. Truth has
>> no quota and is eliminated; her 2100 votes pass to Lee; and Lee is elected.
>> That kind of pathology could cause a civil war.
>>
>> Part of this is the LNH property of STV biting us. With a system that can
>> see past the top choice, you'd have 2100 votes which were essentially
>> half-Douglass-half-Lee. In systems like Schulze STV, that would at least be
>> helping Douglass, so the Lee voters would have to think twice about the
>> tactic. And in systems like RRV, that wouldn't be any more likely to elect
>> Lee than Lee's 1100 honest votes, so the tactic would be useless.
>>
>
> Couldn't a more complex variant work in Range as well? Say you have honest
> voters who prefer A > B > C, and some dishonest voters who prefer  D to win.
> They could either vote D first and play honestly, or they could vote A > D >
> ...
>
> By voting A > D, their tactic is to get the second seat and also to
> downweight the A > * ballot in doing so. They know that they can get D
> anyway, but if they poison the A > * ballot with D, they can both have their
> cake (get D) and eat it too (make the honest voters' latter choices count
> less).
>

I think this would not work. They'd be reducing B and C's scores on the A
ballot, but they'd be increasing the overall weight of the A ballot. In RRV,
this would balance out perfectly, neither helping nor harming B and C.

In any system which did not use averages - anything with medians or such -
the balance would not be perfect, but I suspect it would generally be good
enough.

JQ


> If there are only two seats, this has no point. If there are many, it might
> pay for a substantial fraction of the D-voters to vote A > D so as to weaken
> the competitions for later seats.
>
> Of course, the tactical voters would have to decide whether they get most
> bang for the buck by voting D, A>D, or a combination of the two. It might
> turn out that the downweighting makes the strategy suboptimal in any case.
>
>
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