[EM] A pairwise elimination satisfying SFC,SDSC

Forest Simmons fsimmons at pcc.edu
Mon Apr 2 15:19:47 PDT 2001

Mike O. recently reminded us that it seems impossible to get IRV
supporters to budge on anything. I think it is a sign of insecurity.

Before Mike reminded me of that, I was thinking of a method based on
preference ballots that might have some of the same psychological
attraction as IRV and still satisfy the SFC and SDSC.

Here it is:


In each round of the runoff either the candidate with the greatest number
of last place votes or the one with the next to greatest number of last
place votes is eliminated, whichever loses in the pairwise comparison of
the two.


This method satisfies SFC because a CW will never lose a pairwise
comparison, and therefore never be eliminated.

This method satisfies SDSC because if a majority prefers A over B, then
truncation at the level of B will insure that B loses as follows:

Candidate B will come up for elimination before A does since B will have
more last place votes than A. If B is not already eliminated before
the time that A is considered for elimination, then B
will lose to A, and thereby be eliminated, QED. 

As far as I know this is the only method based on preference ballots that
meets SFC and SDSC while totally avoiding the issue of cycles.

Do you think this method has a better chance among IRV supporters than 
Approval or CR ?

If so, then shouldn't we offer it to them as an alternative?


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