Ted,
In response to Abdul asking:
What if we had IRV with Approval? What is that called?
You wrote:
ERIRV(whole):
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.
http://lists.electorama.com/pipermail/election-methods-electorama.com/2005-May/015981.html
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
repeat.
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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