[EM] Just In Time Withdrawal - JITW

[Donald E Davison]

Tom Round wrote:
>Sorry to be so dumb, but I'd been unable to subscribe to this list for
>about 18 months, and I see terminology has moved on whilst I've been frozen
>in the ice ... What does "JITW" stand for?
>
>PS. Suggestion: perhaps the list-owner, and other list participants who are
>willing, could incorporate a http:// link to the FAQ web-page for this list
>into their e-mail sigs ...? Just a thought.
>
- - - - - - - - - - - - - - - - - - - - - - - - - - - -
Dear Tom Round,

     In the event no one has yet informed you, JITW stands for Just in time
withdrawal.
     The following is some text written by Steve Eppley that deals with JITW.

Donald
- - - - - - - - - - - - - - - - - - - - - - - - - - - -
<snip>
My humble offering is to move the complexity out of the method, into
the laps of the dominant or near-dominant candidates (or the laps of
the sponsors of dominant ballot propositions, when we vote on rival
propositions).  Simply precede Instant Runoff (or plain Condorcet)
with two simple steps:

   1. After the voters cast their preference order ballots, publish
      the preference orders (electronically, on the internet).

   2. After step 1 is performed, allow candidates a short period of
      time (perhaps a week) to voluntarily withdraw from contention
      before the final result is tallied.  (No candidate can be
      forced to be a spoiler.)

   3. Tally the preference orders using Instant Runoff (or plain
      Condorcet), ignoring rankings of the withdrawn candidates.

To use Bruce Anderson's syntax, this method is named JITW//IRV (or
JITW//Condorcet).  JITW stands for "just-in-time withdrawal."

I mention JITW//IRV before JITW//Condorcet because JITW//IRV is a
reasonable compromise which can be struck with IRV advocates.  It
satisfies the criteria stated by IRV advocates at least as well as
plain IRV, and satisfies the criteria of pairwise advocates, without
adding significantly to the complexity of the IRV description.

I don't think there can be any serious argument against step 1,
regardless of the overall system, wherever ballots are machine-
readable.  It would allow the performance of the system to be better
analyzed.  It would provide more information about the voters'
preferences, which is the point of having people vote.  The omission
of this step is a significant oversight in the IRV initiatives
currently being promoted.  In the context of JITW, the ballot
information would help candidates decide whether or not to withdraw,
and would help voters decide whether to encourage their favorite
candidates to withdraw.

We've seen many examples posted here showing how, given plain Instant
Runoff, sincere votes such as ABC can backfire and elect C.  (C could
have been defeated if those voters had instead ranked B first.)
JITW//IRV would allow candidate A to respect the ABC voters' intense
preference for B over C, whenever they are intense enough, by
withdrawing from contention, without forcing those voters to
insincerely rank B first.

France already uses a related system.  They use a runoff system in
which any candidate with at least 12.5% of the vote in the primary
qualifies for a runoff.  They permit candidates to voluntarily
withdraw before the runoff, and that is exactly what most candidates
do when they predict they'd otherwise be spoilers.

Some people may argue that JITW// would create the possibility of
backroom deals among the candidates which would defeat the will of
the voters.  It should be pointed out that most candidates would be
forcibly eliminated by the //IRV (or //Condorcet) anyway; only
"dominant" candidates would be able to improve the outcome by
withdrawing.

Also, the proportional representation systems (and non-proportional
multiparty systems) which many people advocate have an even worse
backroom deal problem, after elections when a majority coalition is
being cobbled together to run the legislature and (maybe) select the
prime minister: In that situation each legislator has a full vote to
sell as s/he pleases, whereas in JITW// systems a withdrawing
candidate is merely stepping out of the way of voters' preferences.

JITW guarantees that no candidate can be made into a spoiler against
his/her will.  (If I correctly understand the Gibbard-Satterthwaite
"manipulability" theorem, only methods which permit just-in-time
withdrawal rigorously satisfy this "no spoiling" criterion.  One
might go further and say that JITW moots the theorem, since the order-
reversal tactic backfires by giving some power to one's "greater
evil": the power to undo the effect of the reversal.)  So given JITW
there is no deterrent against competing and no need for a party to
nominate only one candidate.  (Hence partisan primaries would be of
little importance.  They'd still be of utmost importance given plain
IRV.)

JITW also allows any voter to vote sincerely, with no strategic
dilemma, whenever she trusts her more preferred candidate(s) to
withdraw when necessary to defeat her least preferred candidate(s).
(Given JITW//IRV or JITW//Condorcet, she can also safely rank her
more preferred candidates ahead of the others whenever she believes
they would surely lose anyway given IRV or Condorcet.)  Presumably,
questions about the circumstances in which candidates would withdraw
would be asked during the pre-election campaigns, and voters would
tend to downrank candidates whose answers are troubling.

JITW//Plurality, clearly the simplest of the plausible methods, would
also work well whenever supporters can trust their more-preferred
candidates to withdraw when needed to defeat their less-preferred
candidates.

* * <snip>
---Steve     (Steve Eppley    seppley at alumni.caltech.edu)