Ranked Choice Including Pairwise Elimination: Difference between revisions
Ranked Choice Including Pairwise Elimination (view source)
Revision as of 05:11, 29 March 2022
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The RCIPE STV method modifies the [[Single transferable vote|Single Transferable Vote]] (STV) method in the following ways:
* During each round of counting either one candidate is elected, or one candidate is eliminated, but not both in the same round.
* At the beginning of each counting round, the transfer count for each candidate is reset to zero.
* At the beginning of all the counting rounds, each ballot has an influence amount equal to one vote. During later counting rounds a ballot can have a reduced influence amount that is expressed as a decimal number that can range from zero up to one. If a law makes it illegal for a ballot to have an influence amount other than zero or one, the counting process can be modified as described below.
* When counting a ballot, the ballot's influence amount is added to the transfer count of the candidate who is ranked highest after ignoring the marks for already-elected and already-eliminated candidates.
* If a ballot ranks two or more candidates at the same preference level, and if there are not any remaining (not-yet-elected and not-yet-eliminated) candidates ranked higher on this ballot, then this ballot's influence amount is equally split among the remaining candidates who are ranked at that shared preference level. For example, if a ballot ranks candidate A highest, and ranks candidates B, C, and D at the next-highest level, and if candidates A and B have been elected or eliminated, then half of this ballot's influence amount transfers to candidate C and the other half transfers to candidate D.
* At the end of a counting round, the candidate with the highest transfer count is elected if that candidate's transfer count equals or exceeds the required quota count. The quota count is different for each counting round. Any reasonable formula for quota counts can be used.
* If all the candidates have transfer counts that are less than the quota number, and if there is a pairwise losing candidate during that round, the pairwise losing candidate is eliminated. During pairwise counting all the ballots are counted, but some ballots can have reduced influence.
* If a counting round does not elect a candidate and there is no pairwise losing candidate, then the candidate with the lowest transfer count is eliminated.
* When a candidate is elected, all the ballots that contribute full (not-reduced) influence to the candidate's transfer count have their influence reduced to the candidate's excess transfer count divided by the transfer count, where the excess transfer count equals the candidate's transfer count minus the quota count. Of course decimal calculation results are rounded down, not up.
* When a candidate is elected, all the ballots that contribute reduced influence (less than full influence) to that candidate's transfer count are reduced to have zero influence in future counting rounds.
If decimal influence amounts are not allowed, the following modifications can be used:
* The influence amount of the ballots that
* Ballots on which more than one remaining candidate shares the same highest ranking level are uniformly allocated to the remaining shared-level candidates. This means that the example above involving candidates A, B, C, and D would give half the ballots that have an influence count of one (during that round) to candidate C, and would give the other half to candidate D.
[[Category:Sequential loser-elimination methods]]
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