Dominant mutual third set

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The dominant mutual third set (DMT set) is a set of candidates such that every candidate within the set pairwise-beats every candidate outside the set, and more than one-third of the voters prefer the members of the set to every non-member of the set.

It was first defined by James Green-Armytage as a more particular version of the mutual majority set.[1]

One implication is that when all but one candidate in the DMT set is eliminated, the remaining candidate will be a Condorcet winner and have over 1/3rd of all 1st choice votes. This is notable in the context of IRV because any candidate who has over 1/3rd of the active votes in any round of IRV is guaranteed to be one of the final two remaining candidates if the IRV winner is found by eliminating candidates until only one remains (since they are guaranteed to be one of the top two candidates in every round, since at most any two other candidates could each have just under 1/3rd of the active votes, or only one other candidate could have over 1/3rd of the active votes), and any candidate who pairwise beats all others must as a consequence win the final round of IRV against the other final remaining candidate, since that is just a pairwise matchup between the two.

Instant-runoff voting always elects a winner from the smallest dominant mutual third set, just like it does from the smallest mutual majority set. Chris Benham later determined that IRV and Smith,IRV also meet dominant mutual third burial resistance (DMTBR):[2] raising a candidate not in the smallest dominant mutual third set cannot make that candidate the IRV winner.

It can be proven that several other Condorcet-IRV hybrid methods pass dominant mutual third burial resistance. For example, with Benham's method, since at least one member of the smallest DMT set is guaranteed to be one of the two final remaining candidates after eliminating the rest, there is no incentive for a voter who honestly prefers that DMT member over the other final remaining candidate to not vote that preference i.e. the same incentive for honest voting exists as if it was a runoff.

References[edit | edit source]

  1. James Green-Armytage (2004-06-06). "IRV vs. approval: dominant mutual third". Election-methods mailing list archives.
  2. Chris Benham (2008-11-25). "Re: Why I Prefer IRV to Condorcet". Election-methods mailing list archives.