[EM] IRV is best method meeting 'later no harm'?

[Chris Benham]

Steve Eppley wrote (26 Nov 2009):
"Can it be said that Later No Harm (LNH) is satisfied by the variation of
IRV that allows candidates to withdraw from contention after the votes are
cast?"

No. Take this classic (on EM) scenario:

49: A
24: B
27: C>B

A is the normal IRV winner, but in the variation you describe C presumably
withdraws causing B to win.

49: A
24: B>C
27: C>B

If the B supporters instead of truncating vote B>C then C wins. Assuming
C accepts the win the B voters have caused B to lose by not truncating, a 
clear failure of Later-no-Harm.

Steve wrote:
"Since IRV is said to satisfy LNH, then one must say Plurality Rule
satisfies LNH too, because Plurality Rule can be viewed as just a
variation of IRV with a smaller limit (one candidate per voter)."

Yes, and I did. I listed FPP ("First-Preference Plurality" or more traditionally
"First Past the Post") as a method that meets Later-no-Harm.

I understand that in the US the Alternative Vote is called IRV, but that sometimes
various inferior approximations are given the same label.

Chris Benham


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