[EM] Why I think IRV isn't a serious alternative KD

Kristofer Munsterhjelm km-elmet at broadpark.no
Sun Dec 21 01:31:35 PST 2008

James Gilmour wrote:
> Kevin Venzke  > Sent: Tuesday, December 16, 2008 1:49 PM
>> The reason I believe LNHarm is more valuable than 
>> monotonicity is that when a method fails LNHarm, the voter is 
>> more likely to realize in what insincere way to vote 
>> differently, in order to compensate. When a method fails 
>> monotonicity, a voter will rarely know to do anything 
>> differently because of it.
> LNH is important to ordinary electors, as I have explained in a
> recent  post, at least where the voting system is susceptible to LNH
> effects. If the vote counting method is not LNH-compliant, electors
> are likely to vote strategically in an attempt to avoid or
> mitigate the effects of LNH-failure or to try to gain some real or
> imagined advantage from its effects.
> Monotonicity, or more specifically, the lack of monotonicity, is of
> no  importance whatsoever in public elections because neither
> candidates nor voters can exploit it. It would be "nice" if the vote
> counting system were monotonic, but we cannot have
> monotonicity AND some of the other criteria we consider desirable.
> For  example, monotonicity and later-no-harm are incompatible in
> IRV and STV-PR. Of the two, LNH is important - non-monotonicity is
> irrelevant.

We can't have both LNHs, mutual majority, and monotonicity (by Woodall). 
FPTP has LNH* (simply because later choices are ignored) and 
monotonicity. IRV has LNH* and mutual majority, but not monotonicity.

I'd say that IRV's monotonicity problem is indeed a problem, because 
it's so pervasive. Just look at Yee diagrams. On the other hand, I'm not 
unbiased, and so I may be saying that because it's "unaesthetic".

In any case, it may be possible to have one of the LNHs and be monotonic 
and have mutual majority. I'm not sure, but perhaps (doesn't one of DAC 
or DSC do this?). If so, it would be possible to see (at least) whether 
people strategize in the direction of early truncation by looking at 
methods that fail LNHarm but pass LNHelp; that is, Bucklin. Was bullet 
voting pervasive under Bucklin?

Unfortunately, no method that passes only LNHarm has been used, so we 
can't do the same there (to see if there was pervasive random filling in 
that case).

We can stil get some idea of how easily voters would strategize by 
looking at Bucklin, though; or for that matter, at ranked voting methods 
that fail both LNHs. Schulze's used in some technical associations 
(Debian, Wikimedia), and, although I don't have raw voting data, they 
seem to be mostly honest. The Wikimedia election had no Condorcet cycles 
down to the sixth place, for instance.

> James Gilmour
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