[EM] questions about SF's version of IRV

[Bart Ingles]

sdfgdfg sdfgdfs wrote:
> 
> Hello, I am wondering what you think of the IRV voting
> system being implimented in SF, specifically:
> 
> It seems that voters will be only allowed to rank
> their top three choices, instead of ranking the whole
> field? Does this make a difference? If so how would
> this type of difference effect a race with 5
> candidates and an incumbent, vs. 20 candidates in a
> non incumbent race? Is it theoretically possible if
> the 20 candidates formed 3 "alliances" which only
> voted for each other, then a 51+ percent majority
> could not be achieved?

That was my understanding.  Of course, the definition used for
"majority" under any form of IRV is pretty weak to begin with-- it
basically means that the Condorcet loser should be eliminated.  In other
words, if more than half of voters include in their rankings a vote for
the eventual winner, then you can say that a majority of voters
preferred the winner over at least one other candidate.  

So with a ten candidate field, the "majority" winner would be preferred
head-to-head over at least one other candidate (even though the
remaining eight candidates may have head-to-head majorities over the IRV
winner).

But if you limit the choices to three, then even this limited definition
only works for four or fewer candidates.

> Is anyone aware if the SF system will allow people to
> only choose one choice, I guess what is called
> "truncation"?

I think that's probably the case.  The only areas I have heard of that
require full ranking are ones that can actually accomodate full
ranking.  I don't think SF would dare try to force people to use all
three slots.  The resulting spoiled ballots would look bad.

> Keep in mind that this would be a very first try for
> IRV, so even with the possibility open for strategic
> voting people would very likely not know what to do
> anyways.

There's nothing wrong with strategic voting under IRV-- it actually
helps overcome the method's shortcomings, just as strategic voting does
under simple plurality voting.  Or most other methods, for that matter.