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

[Peter Zbornik]

Dear all,

do any one of you know of any implemenation 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 prefered 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 algoritm above?
It seems to be a useful feature, when the voter does not want to be forced
to prefer one candidate over another.

I discussed this issue on the EM list several years ago.
No implementation was then available, so I give this a try now again.

Best regards

Peter Zbornik
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.electorama.com/pipermail/election-methods-electorama.com/attachments/20160601/69de8119/attachment.htm>