[Election-Methods] Unnecessary voting method?

Steve Eppley SEppley at alumni.caltech.edu
Tue Dec 18 06:22:03 PST 2007


Stéphane's latest example (immediately below) is very different from his 
earlier example that I quoted (further below) which he tallied using a 
voting method he called "Repetitive Condorcet (Ranked Pairs (Winning 
Votes)) Elimination."  His earlier example had no "approval cutoffs" and 
his latest example appears to have no connection to Ranked Pairs or 
Condorcet.  Thus he hasn't provided a basis for claiming my comment was 

My advice to Stéphane for when he sobers up (just joking) is to reread 
his earlier example and then provide a clear definition of the 
"Repetitive Condorcet (Ranked Pairs (Winning Votes)) Elimination" 
method, or a link to its definition, so we will know what voting method 
he was writing about.  Based on the name he gave it and from his earlier 
example, it appears (to me, at least) to be the method that iteratively 
eliminates the candidate ranked last by MAM until one remains.

The thrust of my comment was that since MAM satisfies Peyton Young's 
LIIA criterion, it follows that MAM elects the same candidate as the 
more complex voting method that iteratively eliminates the candidate 
ranked last by MAM until one candidate remains.  Was Stéphane claiming 
this is wrong, when he wrote that my comment was wrong?

Second, I do not understand what he meant where he wrote, "meaningless 
winners which could not get elected with SPPA in the end."  I suspect it 
is not relevant to the comment I made.

Stéphane Rouillon wrote:
> First Steve's comment is wrong as shown below: A > B > C.
>> 33: A > B | C
>> 31: B > C | A
>> 33: C | A > B
>> 3:   B | A > C
>> C is eliminated with 33 votes as support.
>> B is eliminated with 34 votes as support.
>> A is last eliminated but receives no rallying voters and finishes 
>> with 33
>> votes as support.
>>   B wins.
> Second, as written before, scores or supports matter, not meaningless 
> winners which could not get elected with SPPA in the end...
> S.Rouillon
> Steve Eppley a écrit :
>> Hi,
>> Assuming I'm correctly understanding a voting method Stéphane 
>> Rouillon used in a recent message (excerpted below), which he called 
>> "Repetitive Condorcet (Ranked Pairs(Winning Votes)) elimination," it 
>> is unnecessarily complicated because it chooses the same winner as 
>> Ranked Pairs(Winning Votes), which of course is simpler.
>> Ranked Pairs(Winning Votes), also known as MAM, satisfies H Peyton 
>> Young's criterion Local Independence of Irrelevant Alternatives 
>> (LIIA).  One implication of LIIA is that elimination of the 
>> last-ranked candidate(s) does not change the ranking of the remaining 
>> candidates.
>> By the way, a different criterion has been masquerading as LIIA in 
>> Wikipedia.  Peyton Young defined the real LIIA in his 1994 book 
>> Equity In Theory And Practice (if not earlier).
>> --Steve
>> --------------------------------------
>> Stéphane Rouillon wrote:
>> -snip-
>>> Let's try a counter-example:
>>> 3 candidates A, B, C and 100 voters.
>>> Ballots:
>>> 35: A > B > C
>>> 33: B > C > A
>>> 32: C > A > B
>>> Repetitive Condorcet (Ranked Pairs(winning votes)  ) elimination 
>>> would produce
>>> at round 1:
>>> 68: B > C
>>> 67: A > B
>>> Thus ranking A > B > C
>>> C is eliminated.
>>> at round 2:
>>> 67: A > B is the ranking
>>> B is eliminated
>>> at round 3:
>>> A wins.
>> -snip-
>> ----
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