Bryan Mills bmills at alumni.cmu.edu
Tue Jan 31 21:02:47 PST 2012

> > Why STV? The original poster wanted elected representatives to have votes
> > proportional to their electoral support yes? There's no need for
> fractional
> > transfers from elected candidates then.
> > >
> >
> > IRV is a form of STV, but it's not my favorite.  Some of the other STV
> > methods (e.g. Schulze-STV and CPO-STV) tend to produce better
> eliminations.
> >
> > But the question of why not STV is a good one.  Several reasons.
> >
> > STV requires much more work on the part of the voter - ranking all the
> way
> > down to a candidate likely to be elected, instead of just one.  That
> > probably means a much larger ballot and/or an arbitrary cutoff between
> > ballot-candidates and write-in candidates.
> >
> dlw: If the number of possible rankings is the number of seats + 2 then
> it's not too bad.  And nobody would be forced to rank umpteen candidates,
> so the low-info voters could just vote for their favorite candidate.

The number of possible rankings is quite a lot larger than S+2.  Even if
you don't transfer votes from elected candidates, there are still C-S
candidates eliminated -- so you'd have (C choose C-S-1)*(C-S-1)!
distinguishable rankings, and even more if you allow equal rankings.  The
only way out seems to be to pre-filter the set of candidates, so you
basically have to drop to approval voting at some point --
candidate-registration petitions and the like -- and then we're back to an
arbitrary cutoff.

Partial rankings might be workable in a weighted-seat STV variant, though.
 If a vote only transfers in case of elimination (and not in case of
surplus), one would only need to rank candidates down to the first
candidate sufficiently likely to be elected, and you could split the ballot
into manageable chunks by party.  Determining a suitable cutoff candidate
still has a cognitive cost, but it probably wouldn't be that bad in

But if we assume that partial rankings are effective, there's still the
strategy/computation tradeoff to deal with: allowing truncated ballots
still doesn't help with favorite-betrayal, and STV variants less
susceptible to favorite-betrayal are also less susceptible to efficient

> The STV variants that are less strategy-prone are computationally
> > inefficient, and even those are not strategy-free.
> >
> > And perhaps most importantly, the more resistant an STV method is to
> > strategy, the more complicated it is to explain and understand.
> >
> > As deterministic methods go, I do like STV methods; but DS fixes a lot of
> > the worries I have about them.
> >
> One could also apply the same sort of approach to simplifying STV with the
> initial treatment of all of the rankings as approval votes to get the
> number of candidates down to N+2, where N is the number of seats.
> As with IRV, it's easier to explain STV when there's relatively few
> candidates to eliminate.  And, it'll mitigate the strategy effects, which
> have to be examined more closely.

The initial treatment of rankings as approval votes introduces some other
problems, though.

With an explicit "approval threshold" in the ranking, it induces a
substantial cognitive cost on the voter (determining the approval threshold
With an implicit "first-preference" approval, it has the same problem as
traditional STV (i.e. IRV), namely of unduly rewarding favorite-betrayal.
With an implicit "all-ranked" approval, the overall system would likely
violate later-no-harm with much higher frequency; by expressing a
preference between two dispreferred candidates one might unintentionally
put the higher of the two in contention.

It may well be that these issues are all less severe than in the
deterministic alternatives to STV, but I still think they're enough to
merit consideration of nondeterministic alternatives.
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