Runoff vs IRO

[David Marsay]

A thought on:
> From:          Mike Ositoff <ntk at netcom.com>
> Subject:       Runoff vs IRO

> Let me re-copy the example I posted before, where IRO fails
> badly but Runoff doesn't:
> 
>  60  70  100  83  75
>   A   B    C   D   E
>   B                D

My 'normal' model of voting is a text-book 1-D spatial one, with 
issues from left to right. In Mike's example I imagine that parties B 
and D would tend to move their posture towards C to increase their 
share of the vote. C could not defend against this, and so would get 
'squeezed out'. I certainly couldn't fit UK parties to this model.  I 
would expect A and E to move 'in' too.

> When the candidate support tapers toward the extremes, as
> it will be if the voters are distributed normally (in both
> senses of the word), and when the smaller outer candidates
> are still big enough to tip the scales among the inner candidates
> when their transfers go inward after elimination, then
> IRO will screw up in this way every time.

But how common is this? Downs has an economic theory of voting, in 
which candidates/parties attempt to maximize their support. Does this 
only apply to the UK?
 
> If elimination starts near the middle, and the biggest candidates
> are more extreme, than in that special situation it's possible
> to contrive an example where IRO elects a CW but Runoff doesn't.

Not contrived - look at the UK!

> It's necessary to have a candidate next to the CW who is
> even smaller than the CW, who at the start gives his transfers
> to the CW. 

We have single-issue parties from time to time (eg., the Greens) and 
plenty of independents who fit this description.

>Runoff fails to elect the CW in that example because
> the biggest 2 candidates are extremes:
><SNIP>
> 
> This is, by comparison, a special trick example, which
> makes IRO do better than Runoff at electing a CW.

In the UK, the biggest parties are often extremes. No trick!

> 4. IRO, but not Runoff, can fail to elect a CW who is everyone's
> exclusive 1st or 2nd choice.
(Since corrected by Mike)) 
> Runoff is better than IRO. Regrettably IRO is being proposed
> in some communities to replace Runoff.
> 
> No on Measure F in Santa Clara.
Santa Clara is not like the UK! Maybe there is no universal best 
method. Just a universally worst one: FPP!
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