two-round system, and may produce different results to instant-runoff voting. A variant of the contingent vote, called the supplementary vote, is used to elect some mayors in the United Kingdom. Another variant elects the President of Sri Lanka. A criticism of this method is that "it requires two polls, and gives opportunity for intrigue of various kinds." Non-voting systems Louisiana and nonpartisan blanket primary systems In the United States, the Louisiana primary, introduced in Louisiana for partisan state elections in 1975 and federal elections in 1978 (with a brief return using a closed primary system in 2010), is virtually identical to the two-round system. Instead of the standard American system of primary elections to choose each party's candidate, followed by a general election contest between the winners of the primaries, the Louisiana primary allows voters to select any candidate, regardless of party affiliation. Candidates receiving an absolute majority of the votes cast are declared the winner. Otherwise, the two highest vote-winners in the first round—in effect, the first round of a two-round system—are then the only candidates whose names appear on the ballot at a runoff election, effectively requiring one candidate to win an absolute majority to take office. The key difference between the Louisiana primary and a typical two-round system is that political parties do not select the individuals using their party labels; rather, candidates can self-identify using the label of their preferred political party (or no party at all). The state of Washington adopted a system similar to Louisiana's in 2008, which came into effect in 2010 after legal difficulties. California approved a similar system in 2010, coming into effect for the 36th congressional district election in February 2011. The system used in Washington and California is referred to as the nonpartisan blanket primary or top-two primary system. Like the Louisiana primary, candidates self-select their party label on the ballot rather than being nominated by a particular political party. The main difference between a nonpartisan blanket primary and either a standard two-round system or the Louisiana primary is that a second round of voting is required, even if a candidate wins an absolute majority of votes in the primary. Two-party-preferred vote In Australian politics (predominantly in the lower (senatorial/house) level(s)), the two-party-preferred vote (TPP or 2PP), is the result of an election or opinion poll after preferences have been distributed to the highest two candidates, who in some