Runoff vs IRO

David Marsay djmarsay at
Fri Oct 16 04:22:13 PDT 1998

A thought on:
> From:          Mike Ositoff <ntk at>
> 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:
> 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!
Sorry, but apparently I have to do this. :-(
The views expressed above are entirely those of the writer
and do not represent the views, policy or understanding of
any other person or official body.

More information about the Election-Methods mailing list