[EM] Free riding

[Juho]

On Sep 3, 2008, at 18:06 , Jonathan Lundell wrote:

> On Sep 3, 2008, at 12:28 AM, Juho wrote:
>
>> I hope this speculation provided something useful. And I hope I  
>> got the Meek's method dynamics right.
>
> Meek completely fixes Woodall free riding. That strategy takes  
> advantage of the fact that most STV methods (to the extent we're in  
> a STV/Meek/etc context) are sensitive to elimination order in how  
> they distribute surpluses. In most other STV methods, if I vote for  
> my first and second preferences AB first, and A has a surplus, then  
> only a fraction of my vote (or a probabilistic whole) transfers to  
> B. But if I rank hopeless candidate Z first: ZAB, then (hopefully)  
> A gets elected before Z is eliminated, and my whole vote goes to B.  
> If Z gets eliminated first, no harm done, I'm left with AB. The  
> hazard, of course, is that so many voters do this that Z gets  
> elected and/or AB eliminated.
>
> Meek cures this entirely via its principle that when Z is  
> eliminated, the ballots are counted *as if Z had never run*.  
> There's no advantage to me in ranking Z first.

Meek's approach to treat all votes in the same way can simply be said  
to be fair.

> Hylland is another kettle of fish. Here, I vote BA instead of my  
> sincere AB, because I "know" that A will be elected without my  
> help, and I can afford to spend my entire vote on B.

I note that you describe Hylland as raking the favourite candidate at  
second place instead of omitting it completely.

Additional free riding scenarios that I tried to cover include also  
e.g. swapping of second and third candidates. The point was really  
that the ordering of all the candidates should be re-evaluated based  
on the estimated probabilities and utilities. Meek like systems will  
give different probabilities than those that allow also Woodall style  
free riding.

> This is only useful, of course, if I'm competing with other A  
> supporters who have some second choice, say AC voters. They will  
> have only a fraction of their votes transfer to C, while I will  
> have my entire vote counted for B because I didn't bother to rank A  
> first, even though A is my first choice (I'd better be very  
> confident).
>
>
>>
> There's a risk to the Hylland strategy, of course, if I make a  
> mistake in judging that A will be elected without my help. Other  
> than that, though, I don't offhand see a way of defending against  
> Hylland free riding.

One simple approach would be to follow a candidate given inheritance  
order (=> trees or explicit candidate given inheritance lists)  
instead of multiple voter given inheritance orders. That would not be  
really STV any more, but at least a reference point to compare with  
(and maybe to start finding some intermediate solutions with optimal  
properties from both sides).

Juho






		
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