[EM] Another IRV failure

[Eric Gorr]

At 9:06 AM -0500 3/10/04, Ken Taylor wrote:
>Basically, what happens if you run the IRV until you have 3 candidates left,
>two of which are tied for last place? If you eliminate both, you're
>basically selecting the winner without looking at any next-choice votes,
>eliminating any benefit that IRV may have, and also picking a winner without
>a "majority" to support him or her. But which one of the last-place
>candidates you choose to eliminate could drastically affect the outcome of
>the election depending on what their next-choice votes were.

Agreed.

IRV supporters seem to agree that only one should be eliminated. Two 
methods I have had mentioned to me were:

1. Simply pick one at random

2. Look at the vote totals for the tied candidates in the previous 
round and eliminate the one with the fewest votes. If they are tied 
in the previous round, go back one more, etc.. until you find a round 
where one has the fewest votes. If you go all the way back, then just 
choose randomly. (This method is used by http://demochoice.org)

What makes this particularly interesting then is that with random 
selections, IRV has the potential to select different winners if the 
same data was run through it multiple times.

Has anyone else come across any 'official' suggestions on how to deal 
with the situation of two or more candidates tied with least votes? 
...not at all an uncommon situation in various data sets I've looked 
at.



The other interesting thing that I have learned about IRV recently is 
the odd way it treats equally ranked ballots.

Basically, if you have a ranking:

   A=B>C

You only contribute .5 to the vote total for A & B in the first 
round...in otherwords, you are punished for believing that two 
candidates are essentially the same and ranking them that way.

This essentially forces the voter into an undesirable situation of 
strictly ranking all candidates being given relief only when they've 
decided they've had enough and truncate the rest...potentially 
loosing their voice in later rounds.



-- 
== Eric Gorr ========= http://www.ericgorr.net ========= ICQ:9293199 ===
"Therefore the considerations of the intelligent always include both
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== Insults, like violence, are the last refuge of the incompetent... ===