[Election-Methods] RE : IRV variant (was 'Median or "ladder" voting with candidates')

Kevin Venzke stepjak at yahoo.fr
Fri Dec 28 07:12:42 PST 2007


--- Chris Benham <cbenhamau at yahoo.com.au> a écrit :
> Kevin Venzke wrote:
> "Try this method (an IRV variant) for example:

By the way, I have named this method "Instant VFA Runoff."

> The voter ranks the candidates. Full ranking or
> truncation are allowed; equal ranking is not allowed.
> Say that X is the number of candidates still in the
> running.
> While X>1:
> If more than half of the original count of ballots
> rank candidate C in the Xth position (i.e. strictly
> last among candidates remaining), then eliminate C.
> Otherwise eliminate the candidate with the fewest top
> preferences as in IRV.
> End while.
> Elect the remaining candidate."
> Chris replies:
> Kevin,
> It seems to me that the specification of "more than
> half the original count of ballots" instead of "more
> than half the unexhausted ballots" causes this to fail
> Independence from Irrelevant Ballots(IIB). What
> compensating advantage do you get by doing that?

Probably nothing tangible, however it seemed safer to specify the method in
this way, and it also seemed to be more obviously in the spirit of finding
the "median" option.

> In the 49A,24B,27C>B scenario you have long held that
> A shouldn't win because A has the only
> majority-strength pairwise loss (to B). And yet no
> candidate is ranked "strictly last" on more than half
> the ballots so nothing stops B from being eliminated
> and A winning just like in regular IRV.

True. I find that very unfortunate about this method.

The two-round version of this method would eliminate B and give B's
supporters the option to vote for C in the second round if they desired.
Also, the first round would use a VFA ballot. B voters would probably find
it attractive to vote against A if they expect that some A voters would
support B over C in the second round.

> I suggest this:
> "Voters rank the candidates,truncation allowed,
> above-bottom equal ranking not allowed.
> Until one candidate remains, eliminate candidates one
> at a time according to these rules:
> (1)If one or more of the (remaining) candidates are
> not ranked (among remaining candidates)above bottom or
> equal-bottom on more than half the ballots that make
> some ranking distinction among remaining candidates,
> eliminate the one of these that is top-ranked (among
> remaining candidates) on the fewest ballots.

I think that this rule doesn't do quite as good a job emulating "ladder
voting," though. With my rule, assuming that voters are sincere and are
laid out (with the candidates) on a one-dimensional spectrum, there should
always be (and can only be) one candidate who can be eliminated due to
bottom rankings. (Unless there are ties.)

> (2)Otherwise eliminate the candidate that is
> top-ranked (among remaining candidates) on the fewest
> ballots.
> Elect the remaining candidate."
> What do you think of that? This meets Sincere Defense
> and keeps IRV's IIB while being much more Condorcetish
> than regular IRV.

It's probably fine. Either method's vulnerability to burial strategy should
be checked out, though.

What I like about mine is that even if burial strategy is used to
disqualify a candidate, it's undisputable that the majority wanted this
candidate to be disqualified given the selection of candidates remaining.
The strategy doesn't seem to take inappropriate advantage of other people's
differently-minded votes.

Kevin Venzke

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