[EM] Spoiler Effect on Wikipedia
dbishop at neo.tamu.edu
Thu Nov 11 16:51:22 PST 2004
Eric Gorr wrote:
> At 6:04 PM +0000 11/11/04, Paul Crowley wrote:
>> On Thu, 11 Nov 2004 12:08:31 -0500, Eric Gorr <eric at ericgorr.net> wrote:
>>> hummm....there appears to be two opposing points of view here.
>>> Chris B. claims that IIA satisfaction does imply ICC satisfaction.
>>> Markus S. claims that it does not.
>> I'm responsible for the edits to that page that make that claim, but
>> if it's wrong please do fix it. Markus S - I'm very surprised that
>> IIA does not imply ICC, could you give an example? I mean the strong
>> version of IIA, the one that no reasonable system satisfies.
> I think you mean the strong version of IIA which no ranked ballot
> method can satisfy.
> Unless I am mistaken, Approval Voting does satisfy IIA
It only meets IIA under the assumption that adding or removing a
candidate does not affect people's votes on the other candidates. This
is an unrealistic assumption.
For example, suppose that the votes are:
Then A has 9 votes, B has 8 votes, and C has 10 votes, so C wins. But
suppose that A withdraws from the election, and so the votes become
5: (spoiled ballots)
1: BC (spoiled ballot)
But I think it's more realistic to expect that the 6 "spoiled ballot"
voters would instead vote for their choice between B and C, which would
make the outcome
12: B (the former A, AB, and B voters)
10: C (the former BC and C voters)
A's withdrawal from the election changes the winner from C to B, and
thus Approval voting fails IIA.
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