CPO-STV, or the Comparison of Pairs of Outcomes by the Single Transferable Vote, is a ranked voting system designed to achieve proportional representation. It is a highly sophisticated variant of the Single Transferable Vote (STV) system, designed to overcome some of that system's perceived short-comings. As in other forms of STV, in a CPO-STV election more than one candidate is elected and voters must rank candidates in order of preference. It is a relatively new system and has not yet been used for a public election.
Traditional forms of STV offer voters incentives to vote tactically in certain circumstances and are said to produce outcomes that do not always accurately reflect the preferences of voters. The reason for these problems is that sometimes STV eliminates, at an early stage in the count, a candidate who might have gone on to be elected later had they been allowed to remain in the contest.
CPO-STV was invented by Nicolaus Tideman and aims to overcome these defects by incorporating some of the positive features of Condorcet's method, a voting system designed for single-winner elections, into STV. CPO-STV works by an exhaustive comparison of the various possible outcomes of an election, in accordance with a particular procedure, in order to determine which outcome best matches the preferences of voters. If used for a single winner election CPO-STV becomes the same as Condorcet's method, in the same way that traditional STV becomes Instant Run-off Voting (IRV).
Read more about CPO-STV: Voting, An Example, CPO-STV and Traditional STV, Potential For Tactical Voting, Impact On Candidates and Factions, Practical Implications