[EM] Tiebreaking in STV: Lundell-LastDifference

[John]

Well yes. Security concerns are separate; just assume the systems and
elections are 100% secure and have perfect integrity when discussing
theoretical methods not related to security and integrity.

We can have a separate discussion on security.  I'll tell you this though:
Hackers are irrelevant.  The main threat is the trusted actors (State,
chain of custody, etc.).

On Thu, May 23, 2019, 7:13 PM Stéphane Rouillon <
stephane.rouillon at sympatico.ca> wrote:

> Thoughts?
> Many, essentially about the reproducibility of results to allow fraud
> detection and prevent hackers. The best approach is to make public all
> ballots so anyone can run a STV algorithm and obtain a identical winners.
> But to certify that results will always be the same we need systematic
> tie-breakers. Thus the paper I presented last year to the MPSA 2018 at
> Chicago. I'll send it when I get my computer instead of the phone...
>
> Regards.
>
> Envoyé de mon iPhone
>
> Le 23 mai 2019 à 12:46, John <john.r.moser at gmail.com> a écrit :
>
> [Not subscribed, CC me on replies]
>
> Jonathan Lundell proposed a rule for tiebreaking in STV:
>
> http://www.votingmatters.org.uk/ISSUE22/I22P1.pdf
>
> 1. Find the first mention of any member of the tied set of candidates on
> each ballot, and calculate the total such mentions for each of the
> candidates, using the transferable weight of each ballot. Ignore ballots
> that do not mention at least one tied candidate.
>
> 2. If all n candidates are still tied, exclude one tied candidate at
> random; finis.
>
> 3. Otherwise, remove from consideration for exclusion the candidate (or a
> random choice from the tied set of candidates) with the highest score from
> step 1.
>
> 4. If only one candidate remains, exclude that candidate; finis.
>
> 5. Otherwise, n is now the remaining number of tied candidates (that is,
> less the reprieved candidates from step 3); continue at step 1.
>
> Basically, when you're trying to exclude candidates in STV and you have
> multiple with the same last-place vote count, use the transfer weights of
> each ballot to perform instant runoff voting between these candidates and
> eliminate the winner from consideration; repeat until you have one
> candidate left.  Eliminate THAT candidate from your STV election.
>
> Lundell cites exclusion of a random candidate in the event of a tie in
> this algorithm.  I propose using the Last Difference method, by Lundell's
> own arguments, and only falling back to random exclusion if that fails.
>
> Lundell's argument for his proposed method is that prior-round tiebreaking
> encourages insincerity, and that Last Difference is superior to First
> Difference by O'Neill's arguments, therefor current-round information is
> even better.
>
> I observe that Lundell's tiebreaker will run first, and so will dominate
> over the fallback.  If strategically targeting Last Difference sacrifices
> Lundell's runoff method, then it will cause losses, and so the strategy is
> unviable; yet Last Difference, when it produces a break, is better than a
> random tiebreaker.  The final fallback would be random.
>
> Last Difference is equivalent to First Difference if the immediate prior
> round was the first difference.
>
> Thoughts?
>
> —John
>
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