[EM] IRV in action.....Improved IRV
Chris Benham
chrisbenham at bigpond.com
Sat Apr 5 07:43:02 PST 2003
James,
In response to a quite plausible, poltical spectrum-based 5 candidate
example of IRV performing poorly , you stated :
"I recognise the problem very well, but what is the practical solution?
I am very sympathetic to Condorcet, but there must be serious questions
about the public acceptability of some of the results it is likely to
produce."
This has prompted me to unveil my idea for Improved IRV (an
IRV-Condorcet hybrid), which I think you might like.
At each step where IRV eliminates the candidate with the lowest tally
of votes, this method instead eliminates, from the set of candidates
whose vote tallies are below average and also not above 25%, the
Condorcet loser . If there is a circular tie for this spot all the tied
candidates are eliminated.
Repeats of this step may result in the field being condensed to three
candidates, each with more than 25% (of the votes left in play). In that
event the Condorcet winner of those three wins.
The big weakness of IRV lies in how it decides which candidate to
eliminate. To me the normal IRV rule is too arbitary. The figure I
use of "above 25%" represents a majority of an "STV quota", and is
therefore almost not arbitary.
I think that this proposed method should appeal to those who like IRV
and are repelled by the pure-Condorcet possibility that a candidate
with almost a majority of first-preference votes could lose to a
candidate who gets no first-preference votes.
The example I referred to at the top:
10: FR > R > C > L > FL
10: R > FR > C > L > FL
15: R > C > FR > L > FL
16: C > R > L > FR > FL
15: C > L > R > FL > FR
13: L > C > FL > R > FR
11: L > FL > C > R > FR
10: FL > L > C > R > FR
With 5 candidates, the average number of votes is 20. So FL and FR
with 10 votes each make up the initial "set of candidates whose vote
tallies are below average and also not above 25%" , so they runoff : FR
d. FL 51-49, so FL is eliminated (and FL's 10 votes are transferred to L.)
New tallies are L: 34 C: 31 R: 25 FR: 10
With 4 candidates, average number of votes is 25%, so FR is only
candidate for elimination so is eliminated.(and FR's 10 votes are
transferred to R.)
New tallies are R: 35 L: 34 C: 31
All tallies are above 25%, so no more candidates for elimination.
C pair-wise beats L 66-34, and C pairwise beats R 65-35 ; so C
wins.
Chris Benham
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