CPO-STV - CPO-STV and Traditional STV

CPO-STV and Traditional STV

The example above clearly illustrates the difference between CPO-STV and traditional forms of STV. Where CPO-STV resulted in the election of Andrea, Carter and Delilah, the same election held under the rules of traditional STV would have resulted in the election of Andrea, Carter and Scott. The differences between CPO-STV and traditional STV are analogous to those between Condorcet's method and Instant Run-off Voting.

The reason Delilah does not win under traditional STV arises from the particular stage at which she is eliminated from the count. Because she is eliminated at this point she cannot benefit from any transfers she might have received at a later stage. In traditional STV the order in which candidates are eliminated during the count is highly influential in determining the final result. Advocates of CPO-STV argue that the sequence in which candidates happen to be eliminated in an election is in fact highly arbitrary and should not influence the result. It was in order to resolve this problem of sequential exclusions that CPO-STV was designed.

The actual effect of sequential exclusions is that the number of first or higher preferences a candidate receives is very important. For example, where a candidate has very few first preferences but is rated second or third by a great many voters it will be difficult for them to be elected. This is because they are likely to be eliminated before any of these lower preferences can be transferred to them. Traditional STV, it is therefore argued, is unfair to compromise candidates, who are likely to have few first preferences but many lower preferences. Other proposed electoral systems that attempt to remove the problem of sequential exclusions from STV are Schulze STV and Sequential STV.