COWPEA Lottery

COWPEA Lottery is a method of proportional representation that uses approval voting, and which uses the lottery from COWPEA (Candidates Optimally Weighted in Proportional Election using Approval voting) to elect candidates individually in a sequential manner, rather than give them weighted power within a parliament as in COWPEA. The following lottery is done k times to elect k candidates:

Pick a ballot at random and list the so-far-unelected candidates approved on this ballot. Pick another ballot at random, and strike off from the list all candidates not also approved on this ballot. Continue until one candidate is left and elect this candidate. If the number of candidates goes from >1 to 0 in one go, ignore that ballot and continue. If any tie cannot be broken, then elect the tied candidates with equal probability.

COWPEA Lottery can be used with the Kotze-Pereira transformation for a score voting variant.

COWPEA Lottery is monotonic, and passes Independence of Irrelevant Ballots (IIB) and the universally liked candidate criterion (ULC). Achieving all three of these together is very rare and seen as a prerequisite for a "Holy Grail" method for a proportional approval or score method. However, these are achieved at the cost of determinism.