[EM] IRV vs Plurality

[Juho]

On Jan 27, 2010, at 8:16 AM, Chris Benham wrote:

> Juho wrote (26 Jan 2010):
>
> <snip>
> "It may well be that this method can be characterized as "not fully
> Condorcet and Approval strategy added". I'm not quite sure that the
> intended idea of "mostly Condorcet with core support rewarded" (= do
> what the IRV core support idea is supposed to do) works well enough to
> justify this characterization and the use of this method (when core
> support is required). There is however some tendency to reward the
> large parties or other core support (as intended) and the behaviour is
> quite natural with some more common sets of votes."
> <snip>
>
> Juho,
>
> I don't see the "IRV core support idea" as a serious part of IRV's  
> motivation.
>
> Rather I see it as reasonable propaganda to on the one hand offer some
> vague philosophical excuse for not meeting the Condorcet criterion,  
> and
> on the other reassure those who are wary of too radical a change  
> (from Plurality)
> that this method will not elect a candidate with very few first  
> preferences.
>

I too believe the requirement of core support is largely used for  
defensive reasons and to make other systems look worse, not that much  
as a true sincere requirement. On the other hand I understand that if  
the current system is a two-party system with single party  
governments, led by one single very powerful person with a long term,  
then in that system those rules tend to have (or at least appear to  
have) about 50% first preference support among the voters, and any  
deviation from this towards having leaders with less first preference  
support and need to cooperate with others in order to be able to rule  
may look like electing "too weak candidates". New ways of working may  
look frightening, and if taken directly into use while parts of the  
system remains in the old mode there might indeed be a transition  
period with problems and confusion, even if the end result would be a  
system that eventually will work better. For these reasons I'd like to  
see good definitions of what kind of core support requirements people  
might have in their mind.

>
> The proper criterion that I see it as being most closely positively  
> linked to is
> Mutual Dominant Third, a weakened version of  Condorcet that says  
> that if more
> than a third of the voters vote all the members of subset S of  
> candidates above
> all the non-member candidates and all the members of S pairwise beat  
> all
> the non-members, then the winner must come from S.

This criterion may well be acceptable to people who are used to  
thinking in terms of Plurality.

>
> Also of course it seeks to put a positive spin on the fact that the  
> candidate
> with the fewest first preferences can't win, even if that candidate  
> is the big
> pairwise beats-all winner.
>
> <snip>
>  51: A>B>>C
>  41: B>>C>A
>  08: C>>A>B
>
>  B>A 61.5 - 59,  B>C 112.5 - 12,  A>C 76.5 - 53
>
> 51% voted A as their unique favourite and 59% voted A above B, and
> yet B wins.
>
> "Yes, and I believe there are more criteria that the method fails. We
> should however from some point of view be happy since the method
> elected B that seems to have 92% core support (maybe this is how I
> defined core support in this method)."
> <snip>
>
> Defining as you do "core support" as approval, what is your  
> objection to
> simpler methods that don't allow ranking among unapproved candidates
> (and so just interpret ranking above bottom as "approval") such as the
> Smith//Approval(ranking) method I endorse?

That is another working method that also at some level rewards core  
support (=approvals). I don't object the method. My first concerns are  
maybe in the direction that you mentioned, "methods that don't allow  
ranking among unapproved candidates". It sounds a bit problematic if  
the voters of the "losing side" would generally not take any position  
on which one of the candidates of the "winning side" should win (if  
they choose not to rank them in order to show their non-approval of  
them). That might lead to not electing a candidate that all like but a  
candidate that the "winning side" internally likes (but that could be  
the worst candidate from the point of view of the "losing side" voters).

>
> Or if you think that it is justified for a candidate with a very big  
> approval
> score to beat a majority favourite with less approval, why not simply
> promote the plain Approval method?

I'm actually not taking any position on if core support should be  
required => just saying what one could do if (this particular kind of)  
core support is required. A two-party system is based on heavy use of  
core support but in a genuine multiparty environment it is more  
difficult for me to find reasons to explicitly require strong core  
support to be present.

The first reason in my head why I don't feel like promoting Approval  
as a good general purpose single winner method for all needs is that  
although it does pretty good work in finding a widely approved  
candidate (when that is what the society wants) when there are two  
leading candidates it has some considerable problems when the number  
of potential winners grows to three or higher (= limited ability to  
express the preferences, voter's dilemma of deciding whether to  
influence one battle or another, and resulting instability of the  
results).

Juho


>
> Chris Benham
>
>
>       
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