[EM] Clone proofing Copeland

[Juho]

On Jan 8, 2007, at 19:38 , Chris Benham wrote:

>
> Juho,
>
> 26: A>B
> 25: C>A
> 49: B>C  (sincere is B>A or B)
>
>
> Juho wrote:
>
>> But I'll however mention some random observations that the  
>> example  that you used made me think.
>> - One could also claim that these votes are a result of strategic   
>> voting but in another way than what you described. Instead of  
>> having  49 voters that strategically changed their vote from B>A  
>> (or B) to  B>C one could have had just one voter that  
>> strategically changed her  vote from C>A to A>B. As a result  
>> numbers 25 and 26 were swapped and  counting the first place votes  
>> gives a different result. The  strategic voter was not able to get  
>> her #1 favourite but she could  easily help her #2 favourite  
>> become elected.
>>
>
> Yes, but that is just an instance of vulnerability to the  
> Compromise strategy common to all methods
> that meet Majority for Solid Coalitions.

All vulnerabilities are of course to be considered. This is like in  
security, the weakest link in the chain sets the level of security.  
Or actually in voting methods one also has to estimate the resulting  
damage and benefits to the strategists (in addition to the  
probability of success etc.).

In this example I tried to emphasize also that counting the first  
priority preferences may leave the second priority preferences still  
vulnerable.

> The Achilles' heel of Condorcet methods in their competition with  
> IRV  is their vulnerability to Burial.

Yes, I guess artificial loops generated by Burial is the weakest  
spot, at least on paper. In the most usual real life situations I  
think Condorcet methods are less vulnerable than the impression one  
gets when reading this mailing list. I mean that e.g. in large scale  
public presidential elections I have not yet seen very credible  
scenarios of a successful strategy. The local political culture is  
also important (e.g. if people tend to think that strategic games are  
essential of if they plan to vote sincerely since that is the only  
respectable way to behave). But in most cases the required level of  
certainty of the way people are going to vote and ability to  
coordinate the strategy are not enough to make the strategy work.  
Also the negative impacts like bad reputation because of the  
strategic actions is a factor.

Probably the vulnerability of Condorcet methods to academic criticism  
in the election reform discussions is larger than their vulnerability  
in practical (public large scale) elections :-). The election reform  
discussions are important as well, but better make the difference  
visible in the discussions.

>> - In addition to strategies one of course also has to pay  
>> attention  to the sincere votes. What would be the best candidate  
>> to elect if  the votes in the example were all sincere?
> Arguably maybe B, but also arguably without rating information we  
> can't tell.

I agree. Missing rating information in the ranking based methods  
forces us to make decisions based on that limited information. My  
concern is that if most people had voted sincerely and the result  
would be as described (electing someone else than B), then the voters  
and media might express dissatisfaction with the used voting method.  
I think protection against the worst failures of voting methods may  
be needed but one must understand that this typically makes the  
behaviour with sincere votes less satisfactory. In the EM list it  
often appears as if people feel that in the situation where  
artificial or natural cycles occur the method should be optimised to  
defend against strategies. I typically represent the viewpoint that  
in most cases optimising the method for the sincere votes and  
electing the alternative with best utility may be enough and the best  
choice.

>> There is thus always a  balance on how much one needs to protect  
>> against strategic voters  since all such changes in the methods  
>> (in most cases) make the  achieved utility with sincere votes a  
>> bit worse.
> I think DMC strikes a good balance.

My understanding is that DMC is a pretty good method. I have often  
spoken also in favour of the very basic methods with good  
justification and utility with sincere votes, like e.g. minmax with  
margins. :-)

Juho Laatu

> Chris Benham
>
>
>

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