[EM] SPE Done Right

[Forest Simmons]

Sequential Pairwise Elimination is a very old traditional Condorcet
efficient method ... one that is very much under-utilized despite the
versatility made possible by the plethora of possible agenda setting
methods.

Here is untweaked SPE for comparison:

1.If only one candidate remains on the agenda, elect that candidate ...
Else let PL be the pairwise Loser of the two least promising agenda
alternatives.
2. Update the agenda by eliminating PL.
3. Repeat steps 1 and 2 until a candidate has been elected.

Compare that with the following tweaked version:

1. If only one candidate remains on the agenda, elect that candidate ...
Else let PL be the pairwise Loser of the two least promising agenda
alternatives.
2. Update the agenda by eliminating PL along with any candidates covered by
PL.
3. Repeat steps 1 and 2 until a candidate has been elected.

This tweak guarantees the winner to be uncovered, thereby rendering the
method immune to the embarrassing complaint that needlessly afflicts almost
all extant election methods: "How come X gets elected when our guy pairwise
dominates X ? Wanna explain that to Judge Judy?"

[Y pairwise dominates X iff Y defeats every friend of X, including X
itself.]

This simple tweak is so easy and essential that there is no good excuse for
omitting it ... which omission could make the deliberative assembly
vulnerable to lawsuits ... now that inexpensive protection is available ...
the same way seat belts became mandatory once they were readily available!

So from now on, whenever I refer to SPE, I mean this version ... SPE done
right!

-fws
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