[EM] IRV vs. Plurality [WAS: Re: To Bill Lewis Clark re: stepping-stone]

Bill Lewis Clark wclark at xoom.org
Sat Jan 24 16:44:19 PST 2004

Bill Clark asked:

> What are some examples where IRV performs worse than Plurality?

Adam Tarr answered:

> 10% FarRight>Right>Centrist>Left>FarLeft
> 10% Right>FarRight>Centrist>Left>FarLeft
> 15% Right>Centrist>FarRight>Left>FarLeft
> 16% Centrist>Right>Left>FarRight>FarLeft
> 15% Centrist>Left>Right>FarLeft>FarRight
> 13% Left>Centrist>FarLeft>Right>FarRight
> 11% Left>FarLeft>Centrist>Right>FarRight
> 10% FarLeft>Left>Centrist>Right>FarRight

> 7%  FarRight>Right>LuckyRight>ModerateRight>ModerateLeft
> 2%  Right>FarRight>LuckyRight>ModerateRight>ModerateLeft
> 4%  Right>LuckyRight>FarRight>ModerateRight>ModerateLeft
> 7%  LuckyRight>ModerateRight>Right>ModerateLeft
> 15% ModerateRight>LuckyRight>ModerateLeft>Right
> 16% ModerateLeft>ModerateRight>LuckyRight>Left
> 15% ModerateLeft>Left>ModerateRight>FarLeft>LuckyRight
> 13% Left>ModerateLeft>FarLeft
> 11% Left>FarLeft>ModerateLeft
> 10% FarLeft>Left>ModerateLeft

These are indeed interesting (and I've reposted them and changed the
subject to make these examples easier to find in the archives, for future
readers.)  They don't strike me as particularly unlikely cases at all, and
it seems I missed examples such as these because I typically don't examine
scenarios with more than 3 or 4 candidates.  (Incidentally, can anyone
provide simpler such examples, that have fewer than 5 candidates?  What's
the simplest such example?)

They still don't necessarily prove that Plurality is superior to IRV,
because there are many cases where IRV outperforms Plurality.  The
question is how many of each type of example exist, and which are more
likely to occur.  Answering that question will require a ridiculous amount
of analysis, so until I get around to that I think I'm going to have to
shelve my argument that IRV is a better choice than Plurality.

*This* is the type of argument I'd be happy to see more of, here.  Actual
realistic examples of where Plurality outperforms IRV -- not name-calling
and snide insults.  Thanks, Adam.

-Bill Clark

Dennis Kucinich for President in 2004

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