[EM] IRV vs Plurality

Chris Benham cbenhamau at yahoo.com.au
Tue Jan 26 22:16:23 PST 2010

Juho wrote (26 Jan 2010):

"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."


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.

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.

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.

 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)."

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?

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?

Chris Benham

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