[EM] Implementation of STV with same/duplicate/tied preference/ranking?

[robert bristow-johnson]

---------------------------- Original Message ----------------------------

Subject: [EM] Implementation of STV with same/duplicate/tied preference/ranking?

From: "Peter Zbornik" <pzbornik at gmail.com>

Date: Wed, June 1, 2016 12:53 pm

To: "EM" <election-methods at lists.electorama.com>

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> do any one of you know of any implementation or software package, which

> deals with tied/same preferences, i.e. a ballot where two candidates have

> the same preference.

>

> Example: Candidates A, B, C, D, E

> Ballots:

> 1: A=B>C>D>E

> 1: A>B>C>D>E

> 1: A>B=C=D>E

>

> The first and the last ballots give the same preference to two candidates.

>

> In "standard" STV, where we only follow the number of "first" preferences.

> after "deleting" elected and eliminated candidates from the ballot, the

> same preference can be resolved during the count by

> a) splitting the tied first preferences into n ballots, each with weight

> 1/n, where n is the number of candidates which at the current stage in the

> count are all most preferred on the ballot. Each of these ballots will have

> a different candidate most preferred and the rest with tied second

> preference.

>

> Example: let's return to the example above. We elect two seats: at this

> point in the count A is elected, none is eliminated. On the last ballot of

> the three ballots above thus B, C and D are tied and all most preferred.

> We thus split the ballot into n=3 ballot, each with weight 1/3 of the

> original weight, with a different candidate most preferred and the rest

> tied:

> Thus the ballot 1: A>B=C=D>E, is at this point in the count, after the

> election of A, treated as three ballots:

> 1/3 B>C=D>E

> 1/3 C>B=D>E

> 1/3 D>B=C>E

> Thus we resolve the tie by simply adding 1/3 of the to the (currently)

> "first" preferences of B, C and D in the count.

>

> This is the only computationally efficient way to resolve ties in STV as

> far as i know.

>

> Does anyone of you know of any implementation of the algorithm above?

> It seems to be a useful feature, when the voter does not want to be forced

> to prefer one candidate over another.
 
i can tell you that when we had IRV in Burlington Vermont 7 years ago, that the voter *was* forced to do that.  equal ranking of candidates resulted in a spoiled ballot for that particular race (which, for us, was only the mayoral
race).





--
r b-j                  rbj at audioimagination.com
"Imagination is more important than knowledge."
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