CPO-STV

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CPO-STV , or Comparison of Pairs of Outcomes by the Single Transferable Vote (pairwise comparison of the results of the transferable individual votes) is a preference voting procedure that is intended to achieve a proportional representation of the election result in the sense of proportional representation . It is a very sophisticated version of the Transferable Einzelstimmgebung (Single Transferable Vote, STV), which will address some of the weaknesses of conventional STV method. As in other forms of STV, more than one candidate is elected in a CPO-STV election and voters can rank multiple candidates according to their preferences. It is a relatively new system that has not yet been used in any public election.

Traditional forms of STV offer voters incentives to choose tactically in certain circumstances. They thus lead to results that do not always correspond to the preferences of the voters. The reason for these problems is that STV sometimes eliminates candidates early in the count who might have been elected later if they had been allowed to stay in the race longer.

CPO-STV was developed by Nicolaus Tideman and aims to overcome the aforementioned disadvantages by including some of the features of the Condorcet methods , which were actually developed for elections with only one winner, in the STV. CPO-STV works with an exhaustive comparison of the various possible election outcomes to determine which outcome best suits voters' preferences. If CPO-STV is used in an election with only one person to be elected, it becomes a Condorcet method, just as the conventional STV becomes Instant Runoff Voting (IRV).

Voting

Each voter ranks the candidates according to their preferences. For example, as follows:

  1. Andrea
  2. Carter
  3. Brad
  4. Delilah
  5. Scott

The exact rules for a respective CPO-STV election decide whether a voter has to rank each individual candidate and whether he can give several candidates the same rank.

Procedure

Determination of the quota

For a CPO-STV election, both the hare rate and the droop rate can be used. However, Tidemann recommends the Hagenbach-Bischoff quota . This is a rational number : the total number of valid votes cast divided by one more than the number of seats to be filled. So the formula is:

Determination of the winner

CPO-STV compares each possible outcome of an election in order to determine the set of winners (candidate constellation) it considers to be the best. The winning result is determined using a Condorcet method. That is, the results are compared one with the other in a series of imaginary duels. Usually there is one outcome that wins each of these tackles. It is this constellation of candidates that is declared elected.

When two results are compared, a special method is used to give each one a score to determine which of the two is the winner. When comparing two results, the following steps exist:

  1. Removal of candidates who do not appear in either of the results: All candidates who do not appear in either of the two results are deleted and their votes are carried over. The vote of each voter who supports an excluded candidate is carried over to his / her closest candidate, provided that it appears in at least one of the two results.
  2. Carryover of surpluses of candidates in both results: Where the number of votes for a candidate is greater than the quota, the surplus above the quota is carried over. However, only the surpluses of candidates who appear in both outcomes are carried over; all other surpluses are ignored. The excess votes can only be transferred to candidates who appear in at least one of the two results.
  3. Adding the total number: After all necessary deletions and transfers have taken place, the number of votes determined in this way for the individual candidates is added in both results. The result is considered to be the score of that result.
  4. Declaration of the winner: The result with the higher number will be declared the winner of this specific comparison.

Sometimes it happens that after every possible outcome has been compared with every other possible outcome, there is no single winner that beats all others. That means there is then no clear Condorcet winner. In such cases a more complicated procedure has to be used that breaks the circular reasoning of the Condorcet paradox so that a clear candidate constellation can be determined as the winner of the CPO-STV election. The exact method of breaking the circular reasoning depends on the type of Condorcet method used. The more sophisticated Condorcet methods include Ranked Pairs (also developed by Tideman) and the Schulze method .

Method of transferring surpluses

Traditional forms of STV differ in the way excess voices are transmitted. Older forms of STV use either a random selection (Hare's method) or a system of transmitting fractions of votes (Gregory method). However, these procedures are quite crude and can encourage tactical voting. Warren's method and Meek's method are more sophisticated. CPO-STV is compatible with all of these methods. It is therefore the responsibility of those who decide on an electoral system which concrete method to use.

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