[EM] IRV vs Plurality

[Chris Benham]

Kathy Dopp wrote (11 Jan 2010):

<snip>

"Plurality is far better than IRV for many many reasons including:

1. preserves the right to cast a vote that always positively affects
the chances of winning of the candidate one votes for

2. allows all voters the right to participate in the final counting
round in the case of top two runoff or primary/general elections"

<snip>

IRV satisfies both of these. 

Regarding the first,assuming that "the candidate one votes for" refers 
to the candidate the voter top-ranks, then top-ranking X in an IRV election 
has the same positive effect on X's chance of winning as does voting for
X in a Plurality election.

It is true that sometimes in an IRV election a subset of X's sincere 
supporters may be able to do better for X by top-ranking some non-X,
whereas in Plurality the best strategy for all of X's supporters is always 
just to vote for X; but that is different.

IRV meets Mono-add-top, which means that a voter who top-ranks X
would never have done better for X by staying home.

Having arrived at the voting booth, the X supporter's overwhelmingly 
best probabilistic IRV strategy is to top-rank X.

Regarding Kathy's second point, IRV voters should be allowed to
strictly rank from the top as many candidates as they wish. 
The voter is then free to ensure that s/he "participates in the final
counting round" by simply ranking all the candidates (or alternatively
if the likely front-runners are known then just make it very likely by
ranking among them).


Chris Benham


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