[EM] Re: Voting Systems Study of the League of Women Voters of Minnesota

Chris Benham chrisbenham at bigpond.com
Wed Jun 8 08:42:03 PDT 2005

In response to  Abdul asking:

>What if we had IRV with Approval? What is that called?
You wrote:

>Equal-Rank [allowed], Instant Runoff Voting, whole [votes counted for
>equal rank].
>In other words, each round of the runoff becomes an approval election
>rather than a single-vote-transfer election.
This isn't the only method that qualifies  "IRV with Approval". As I've 
explained in previous posts, ER-IRV(whole) is much more vulnerable than 
plain IRV  to voters taking advantage
of its mono-raise failure with a Pushover-like strategy.

>> 45:Right=Left>CentreRight   (sincere is Right>CentreRight)
>> 35:CentreRight>Right>Left
>> 20:Left>CentreRight>Right
>> First-preference tallies
>> Right:45       CentreRight:35      Left:65
>> CentreRight has the lowest tally, and so is eliminated then Right wins.
>> This time no coordination was needed. As long as the Right suporters 
>> knew that Right had more first-prefernces than CentreRight, and a
>> pairwise win against Left, then each individual Right supporter got 
>> an increased expectation by insincerely upranking Left from last to
>> equal-first  with no risk.
> This would also work if the numbers 45/35/20 were replaced with 49/48/3.

For this reason I rate  ER-IRV(whole) as worse than plain IRV.

A much better version of  "IRV with Approval" is Approval Elimination 
Runoff  which fixes the elimination schedule by the initial approval 
scores and has a majority stopping rule.
So if the  candidate who is highest ranked (among remaining candidates) 
on the most ballots is so on a majority of the (non-exhausted) ballots, 
then that candidate wins. If not,
eliminate the (remaining) candidate with the lowest approval-score, and 
The version that uses plain rankings ballots and interprets all  ranked 
candidates as approved (not allowing them to enter an explicit approval 
cutoff and rank disapproved candidates)
is  Woodall's  Approval Alternative Vote (ApAV).

Chris Benham



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