CPO -STV, or Comparison of Pairs of Outcomes by the Single Transferable Vote ( Pairwise comparison of results of Transferable Einzelstimmgebung ) is a preferential voting method to realize a proportional representation of the election results in terms of proportional representation. It is a very sophisticated version of the Transferable Einzelstimmgebung ( Single Transferable Vote, STV), which should fix some of the weaknesses of conventional STV method. As in other forms of STV, in a CPO -STV election more than one candidate is elected, and voters can make according to their preferences more than one candidate in a ranking. It is a relatively new system that has yet been applied in any public election.
Traditional forms of STV offer voters incentives to vote tactically under 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 sometimes STV candidates at an early stage of counting eliminated that would have been later, perhaps chosen if they had longer allowed to stay in the race.
CPO -STV was developed by Nicolaus Tideman and aims to overcome the said drawbacks by some of the features of the Condorcet methods, which were originally developed for options with only one winner to be included in the STF. CPO -STV works with an exhaustive comparison of the different possible election outcomes to determine what outcome best suits the preferences of the voters. If CPO -STV used in an election with only one person to be elected, she will become a Condorcet method, as well as the conventional STV to Instant Runoff Voting ( IRV ).
Each voter ranks the candidates according to their preferences. For example, as follows:
The exact rules for a particular CPO -STV election to decide whether a voter must bring every single candidate in the ranking, and whether he can give more candidates the same rank.
Determination of the ratio
For CPO -STV election both the Hare quota and the Droop quota may be used. However Tidemann recommends the Hagenbach -Bischoff quota. This is a rational number: the total number of valid votes divided by one more than the number of seats to be filled. The formula is:
CPO -STV compares each possible outcome of an election to determine the amount of winners to determine (candidate constellation ), which it sees as the best. The winning result is determined by means of a Condorcet method. That is, the results are compared in a series of imaginary tackles one with the other. There are usually a result that wins each of these duels. It is this candidate constellation which shall be declared elected.
If two results are compared with each other, a special method is used to give each a score and thus determine which of the two is the winner. If two results are compared, there are the following steps:
Sometimes it happens that after every possible outcome to every other possible outcome was compared, no single winner that beats all others, it is clear. That there is then no clear Condorcet winner. In such cases, a more complicated procedure must be used which breaks the vicious circle of the Condorcet paradox, so that a unique candidate constellation as the winner of the CPO -STV election can be determined. The exact method to break up the circularity depends on the type of Condorcet method used. Among 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 in which surplus votes are transferred. Older forms of STV either use a random sample ( Hare's method) or a system of transfer of votes fractions ( Gregory method). However, these methods are rather crude and can encourage tactical Select. Warren's method and Meeks method are more sophisticated. CPO -STV is compatible with all of these methods. It is the responsibility of those who opt for an electoral system which specific method to be used.