Consensus on IRV & Runoff?

Wed Jan 30 00:29:08 PST 2002


Steve asked:

Do we have a consensus that the instant runoff vote (IRV) is MATHEMATICALLY
better than the common two step plurality vote (primary) with a follow-up
runoff between the 2 top plurality vote getters? It seems to me that it 
be worse, given that manipulation is bad. As far as I can see, the only
mathematically provable difference between them is that IRV is less
manipulatable. They share the same faults, including non-monotonicity. With 
candidates, I believe they are mathematically identical.

I reply:

Here's a big difference:

With Runoff, if the sincere CW comes in 1st or 2nd as indicated
favorite in the primary, s/he can't lose.

With IRV, it doesn't matter if the sincere CW is the expressed
favorite of more people than any other candidate--s/he can still
be eliminated & lose.

Another difference:

For Runoff, the mathematical strategy isn't too complicated to
write in one line. Cox, in one of his books, describes Runoff's
mathematical strategy for maximizing utility expectation.

It only involves a few summations, maybe 3 or 5.

IRV strategy would be horrendously more complicated. If IRV were
adopted, maybe someone would publish something about it, but
it wouldn't matter to me, since I'd boycott the elections anyway,
and urge others to.

Mike Ossipoff

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