[EM] Consensus?: IRV vs. Primary w/Runoff

Forest Simmons fsimmons at pcc.edu
Wed Jan 30 10:14:02 PST 2002

On Tue, 29 Jan 2002, Bart Ingles wrote:

> Forest Simmons wrote:
> > 
> > Another angle just occurred to me: a simulation of the two step runoff
> > might be preferable to the IRV simulation of the many step runoff.
> I believe this is what is known in Britain as the "supplemental vote". 
> The other problem here is that you need to anticipate which backup
> choice will survive to the runoff round.  Otherwise both of your choices
> could be eliminated at the outset.

I think supplemental vote only allows you to specify a first and second
choice, which wouldn't be enough to simulate two step runoff.

To simulate two step runoff you need the same kind of ballots that are
required for the IRV simulation of sequential runoff.

Here's a summary of why the two step simulation is not as bad as IRV: 

(1) The two step simulation would be much more faithful to actual two step
runoff than IRV is to sequential runoff (for reasons of strategy mentioned
in my first posting on this topic). 

(2) The two step simulation is summable in data structures that grow only
as the square of the number of candidates.

(3) Strategy is easier for two step. (See Mike's posting on this.)

(4) IRV can eliminate a Condorcet Winner even if it has the greatest first
place support.  Two Step cannot eliminate a Condorcet Winner if it is in
the top two, since a CW cannot be eliminated in any head-to-head contest.
(Again, Mike Ossipoff's observation.)

(5) Two Step doesn't lure the voter into voting for fantasy candidates at
the expense of a fully viable compromise. The folly of this is more
transparent in Two Step because its strategy is simpler. In other words,
Two Step doesn't pretend to satisfy the Favorite Betrayal Criterion, but
it is easy for IRV promoters to fool (themselves and other) people into
believing that IRV does satisfy the FBC.


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