Sequential pairwise elimination: Difference between revisions

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(Forest clarified that SPE works in reverse order of the base method. I've updated the description and example to reflect that.)
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'''Sequential pairwise elimination''' is a class of voting methods devised by [[Forest Simmons]]. These methods elect from the [[Banks set]] and thus pass the [[Condorcet criterion]] and never elect covered candidates.
'''Sequential pairwise elimination''' is a class of voting methods devised by [[Forest Simmons]]. These methods elect from the [[Banks set]] and thus pass the [[Condorcet criterion]] and never elect covered candidates.


An SPE method, emulating legislative procedure, works by first determining a base social order by some method (e.g. [[Minmax]] or [[Range]]). Then starting with the winner of that order (or list), compare the current candidate at the head of the list with the candidate next to it. Replace the two adjacent candidates in the list with the candidate who beats the other pairwise, then repeat. The candidate who is left standing at the end is the winner.
An SPE method, emulating legislative procedure, works by first determining a base social order by some method (e.g. [[Minmax]] or [[Range]]). Then starting with the loser of that order (or list), compare the current candidate at the end of the list with the candidate next to it. Replace the two adjacent candidates in the list with the candidate who beats the other pairwise, then repeat. The candidate who is left standing at the end is the winner.


In effect, each candidate can be considered a proposal (a bill or an amendment). The "legislature" repeatedly votes whether to keep the current bill or to replace it with an amended bill. At the end of the procedure, the last accepted amended bill wins.
In effect, each candidate can be considered a proposal (a bill or an amendment). The "legislature" repeatedly votes whether to keep the current bill or to replace it with an amended bill. At the end of the procedure, the last accepted amended bill wins.
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Suppose that the base order is according to [[first past the post]]. Its order is Memphis > Nashville > Knoxville > Chattanooga.
Suppose that the base order is according to [[first past the post]]. Its order is Memphis > Nashville > Knoxville > Chattanooga.


In the first round, we compare Memphis to Nashville. As the table on the right shows, more voters prefer Nashville to Memphis than vice versa. So Nashville survives the comparison and Memphis is eliminated.
In the first round, starting at the end of the list, we compare Knoxville to Chattanooga. As the table on the right shows, more voters prefer Chattanooga to Knoxville than vice versa. So Chattanooga survives the comparison and Knoxville is eliminated.


In the second round, we compare Nashville to Knoxville. As more voters prefer Nashville to Knoxville, Knoxville is eliminated and Nashville stays in the game.
In the second round, we compare Chattanooga to Nashville. As more voters prefer Nashville to Chattanooga, Chattanooga is eliminated in favor of Nashville.


Finally, we compare Nashville to Chattanooga. Chattanooga is eliminated.
Finally, we compare Nashville to Memphis. Memphis is eliminated.


Thus Nashville is the winner.
Thus Nashville is the winner.


In this example, no matter what the order is, Nashville eventually becomes the incumbent and defeats all subsequent challengers in the order. This happens because Nashville is the Condorcet winner and every SPE method passes the [[Condorcet criterion]].
In this example, no matter what the order is, Nashville eventually becomes the incumbent and defeats all subsequent challengers. This happens because Nashville is the Condorcet winner and every SPE method passes the [[Condorcet criterion]].


[[Category:Condorcet methods]]
[[Category:Condorcet methods]]
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