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

[Chris Benham]

Kevin Venzke wrote:

"Try this method (an IRV variant) for example:

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."

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?

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.

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.

(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.

Chris Benham
 



Thu Dec 20 21:43:33 PST 2007 

Hi,

I think an approach towards implementing this kind of
logic in an election with unnumbered candidates would
be to allow voters to torpedo the options they
perceive as furthest from them.

Try this method (an IRV variant) for example:

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.


Imagine that the candidates can more or less be
plotted on a one-dimensional spectrum. Considering
that candidates are more likely to try to stand as
near to the median as possible, and not spread
throughout the space where voters lie, IRV is likely
to eliminate all the median options and end with a
final showdown between two strong candidates who
were able to grab large quantities of "outer" voters.

In this variant method, assuming these two candidates
aren't the preference of the median voter, it is
likely that IRV's two finishers could be the
first two candidates eliminated. Their supporters'
second preferences would very quickly be freed up to
help support candidates closer to the median.
And this process is capable of repeating indefinitely
until the final two candidates are truly those that
came nearest to the median.


This is an "instant" generalization of my two-round
method suggestion where the final round consists of
the top two candidates from the first round,who didn't
receive a full majority of the "against" votes of that
round.

Kevin Venzke






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