[EM] FWD Filling a vacancy in STV:

[Donald Davison]

  ------------- Forwarded Letter ------------
From: steve.chessin at alum.mit.edu
Date: Mon, 10 Nov 2003 10:45:21 -0800
Subject: Re: Filling a vacancy in STV:

There are at least four possible methods of filling a vacancy on a body
that is elected via Choice Voting (STV), of which only two ((c) and (d)
below) arguably preserve the original proportional intent of the
voters:

        a. by appointment by the body (may not preserve proportionality).
        b. by holding a new election to fill the vacancy (but since
           only one seat is being filled, the result may not preserve
           the original proportional intent)
        c. by taking just those ballots that resulted in the election
           of the vacating official, each at the weight it had to elect
           that official, and using them to conduct an IRV election,
           skipping over any candidates who (a) were elected, (b) have
           vacated, or (c) have withdrawn from consideration.  (This
           method is called "countback" by the Proportional
           Representation Society of Australia.)  Of course, one has to
           ask all the potentially eligible candidates if they are
           still interested in serving before conducting the
           countback.  In effect, this method asks the voters who
           elected the vacating candidate, *and only those voters*, who
           their next choice is, and thus arguably preserves not only
           the proportionality of the original result, but the intent
           of the original voters.
        d. by recounting all the ballots from scratch, excluding the
           vacating official as well as any no-longer-willing
           candidates, stopping the count when the first person not
           originally elected is declared elected, and having that
           person fill the vacancy.  (This only partially preserves the
           proportionality of the result, since if this had been the
           original configuration of candidates, one might find that
           one or more of the currently seated officials might not have
           been elected.  If you run the count to completion, you'll
           find out who those people were.)

I personally prefer (c), although CVD's model language at
http://www.fairvote.org/library/statutes/choice_voting.htm uses (d).  I
believe that Voting Solution's ChoicePlus software can handle either
method.

--Steve