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

[Chris Benham]

Forest,
"What nicer distribution can  you think of.."

"Nice" (and "nicer") is a fuzzy emotional/aesthetic term that I might apply to food, music, people etc.
but seems unscientific and out-of-place here (and I'm not sure exactly what it's supposed to mean). 

I can see that such a distribution is more comfortable for methods that try to elect the centrist candidate.

I see IRV as FPP that trades most of its monotonicity criteria (including mono-raise and Participation but 
not mono-add-top, mono-add-plump or mono-append) to gain Clone-Winner and Majority for Solid
Coalitions (and Mutual Dominat Third and Condorcet Loser).

It keeps FPP's compliances with Woodall's Plurality criterion, Later-no-Harm, Later-no-Help and Clone-Loser.

The "representativeness criteria" it meets generally allow for a bigger set of allowable winners than say
the Smith set, and its monotonity failures mean that it chooses a winner from this set a bit erratically.
But I think your use of  the term "pathology" (comparing it to a disease and so something  that is self-evidently
unacceptable) is biased and out of place.

I also think that the argument that IRV makes a good stepping-stone to  PR is strong. Truly proportional 
multi-winner methods meet  Droop Proportionality for Solid Coalitions (equivalent in the single-winner 
case to Majority for Solid Coalitions, aka Mutual Majority.)

Single-winner STV's  virtues of  Later-no-Harm and Clone Independence survive into the multi-winner 
version (which of course meets Droop Proportionality SC), while for multi-winner methods the Condorcet 
criterion and Favourite Betrayal  are both incompatible with Droop PSC.  Also I think Later-no-Harm 
compliance is more valuable for multi-winner methods than for single-winner methods.

Chris Benham



Forest Simmons wrote (Sat. Nov.29):

>From: Chris Benham 
> > Forest,
> Given IRV's compliance with the "representativeness criteria"  Mutual Dominant Third, Majority for
> Solid Coalitions, Condorcet Loser and? Plurality; why should the  bad look of its "erratic behaviour"
> be sufficient to condemn IRV in spite of these and other  positive criterion compliances such as
> Later-no-Harm and Burial Invulnerability?
 
A picture is worth a thousand words.  It shows the actual behavior, including the extent of the pathology.

> > "....in the best of all possible worlds, namely normally distributed voting populations in no more 
> than two dimensional issue space."
> >CB: Why does that situation you refer to qualify as "the best of all  possible worlds" ?
> Three points determine a plane, so we cannot expect a lower dimension than two. What nicer 
>distribution can you think of. than normal?  But any distribution whose density only depends on distance 
>from the center of the distribution would give exactly the same results for any Condorcet method, without 
>making the IRV results any nicer. 

Forest


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