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Schulze STV

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Schulze STV

Schulze STV is a proposed multi-winner ranked voting system designed to achieve proportional representation. It was invented by Markus Schulze, who developed the Schulze method for resolving ties using a Condorcet method. Schulze STV is similar to CPO-STV in that it compares possible winning candidate pairs and selects the Condorcet winner. It is named in analogy to the single transferable vote (STV), but only shares its aim of proportional representation, and is otherwise based on unrelated principles.

The system is based on Schulze's investigations into vote management and free riding. When a voter prefers a popular candidate, there is an advantage to first choosing a candidate who is unlikely to win ("Woodall free riding") or omitting his preferred candidate from his rankings ("Hylland free riding"). Schulze STV is designed to be as resistant to free riding as possible, without giving up the Droop proportionality criterion.

Each voter ranks candidates in their order of preference. In a hypothetical election, three candidates vie for two seats; Andrea and Carter represent the Yellow Party, and Brad represents the Purple Party. Andrea is a popular candidate, and has supporters who are not Yellow Party supporters. It is assumed that the Yellow Party can influence their own supporters, but not Andrea's.

There are 90 voters, and their preferences are

In the STV system, the initial tallies are:

The quota is determined according to Andrea is declared elected and her surplus, , is distributed with

Brad is also elected.

The Schulze STV system has three possible outcomes (sets of winners) in the election: Andrea and Carter, Andrea and Brad, and Carter and Brad. In this system, any candidate with more than the Droop quota of first choices will be elected. Andrea is certain to be elected, with two possible outcomes: Andrea and Carter, and Andrea and Brad.

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