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

[Jonathan Lundell]

On Nov 25, 2008, at 11:25 AM, Markus Schulze wrote:

> Dear Jonathan Lundell,
>
> Greg Dennis wrote (25 Nov 2008):
>
>> I've studied every IRV election for public office
>> ever held in the United States, most of which have
>> their full ranking data publicly available, and
>> every single time IRV elected the Condorcet winner,
>> something I consider to be a good, though not
>> perfect, rule of thumb for determining the "right"
>> winner.
>
> I wrote (25 Nov 2008):
>
>> If I remember correctly, Abd wrote that, in every
>> IRV election for public office ever held in the
>> USA, the IRV winner was identical to the plurality
>> winner. Doesn't that mean that -- when we apply
>> your logic -- plurality voting always elects the
>> right winner?
>
> You wrote (25 Nov 2008):
>
>> Plurality failed in Florida 2000, so we can conclude
>> that "plurality voting always elects the right winner"
>> is false.
>
> And when you apply Abd's claim to your conclusion (that
> the statement "plurality voting always elects the right
> winner" is false), what can you conclude about Greg's
> claim?

Greg concludes that IRV, in practice, tends to elect the Condorcet  
winner. Does he conclude that it must always be so? I don't think so.

Abd says that the IRV winner in these cases was also the plurality  
winner. Again, no claim of necessity.

We might equally well conclude that plurality usually elects the  
Condorcet winner, and that it fails infrequently enough that we don't  
have examples of IRV correcting a plurality error. (Florida 2000 is an  
example of a plurality error that IRV would most likely have corrected.)


My own view is first that we're talking about marginal differences  
here, and that PR vs single-winner elections is of much, much greater  
interest, and second that the interesting difference between  
plurality, IRV and other ranked methods is not in how they count any  
particular profile, but rather in how they influence candidate and  
voter behavior. In the IRV examples that Greg and Abd adduce, we don't  
actually know what the ballots would have looked like if the elections  
had used plurality. The set of candidates might well have been  
different, the nature of the campaigns different, and voter strategies  
different.

Given an IRV election, the question "how would this election have  
turned out if plurality had been used" cannot be answered by counting  
the IRV first choices.