Definite Majority Choice

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Revision as of 14:39, 22 March 2005 by Araucaria (talk | contribs) (The Ballot: changed the default LPG)

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Definite Majority Choice (DMC) is a voting method proposed by several (name suggested by Forest Simmons) to select a single winner using ballots that express preferences, with an additional indication of Approval Cutoff.

If there is a candidate who is preferred over the other candidates, when compared in turn with each of the others, DMC guarantees that that candidate will win. Because of this property, DMC is (by definition) a Condorcet method. Note that this is different from some other preference voting systems such as Borda and Instant-runoff voting, which do not make this guarantee.

The main difference between DMC and Condorcet methods such as Ranked Pairs (RP), Cloneproof Schwartz Sequential Dropping (Beatpath or Schulze) and River is the use of the additional Approval score to break ties. If defeat strength is measured by the Total Approval score of the pairwise winner, DMC finds the same winner as those other three methods [This needs to be verified! --Araucaria 12:22, 21 Mar 2005 (PST)]

DMC chooses the same winner as (and could be considered equivalent in most respects to) Ranked Approval Voting (RAV) (also known as Approval Ranked Concorcet), and Pairwise Sorted Approval (PSA).


The Ballot

Voters rank their preferred candidates, from favorite to least preferred, and may optionally specify an Approval Cutoff.

A Graded Ballot ballot implementation would infer the ordinal ranking from the grades given to candidates, and the Approval Cutoff would be determined with a Lowest Passing Grade option. Voters could grade their choices from favorite (A+) to least preferred (ungraded), and give some or all of their graded choices a "passing grade" to signify approval.

            A    B    C    D    F       +  /  -
      X1   ( )  ( )  ( )  ( )  ( )     ( )   ( )
      X2   ( )  ( )  ( )  ( )  ( )     ( )   ( )
      X3   ( )  ( )  ( )  ( )  ( )     ( )   ( )
      X3   ( )  ( )  ( )  ( )  ( )     ( )   ( )
   Lowest  ( )  ( )  ( )  ( )  ( )     ( )   ( )
   ("C" by default)

A voter may give the same grade (rank) to more than one candidate. Ungraded candidates are graded (ranked) below all graded candidates.

Any candidate at the Lowest Passing Grade or higher is given one Approval vote. Unless explicitly changed, the Lowest Passing Grade is assumed to be C.

No Approval votes are given to ungraded candidates or candidates graded below the Lowest Passing Grade (i.e., C- and lower, by default).

Grades assigned to non-passing (disapproved) candidates help determine which of them will win if the voter's approved candidates do not win.

Adding a plus or minus to a candidate's grade is optional, but enables 16 rank levels (including no rank given).

99% of the time, there should be no need to change the LPG -- with 8 default approved grade levels from A+ to C, there is plenty of room to express relative preferences.

Setting the default LPG at C instead of C-minus allows an indecisive voter to be hesitant about granting approval by initially filling in a grade of C. If after reconsideration the voter decides to disapprove the candidate, the minus can then be checked.

Having an LPG option may be useful as a last resort --- it allows a voter to move the cutoff higher or lower after entering grades if the voter makes a mistake. But it is not necessary to have an LPG option in a first public proposal.


What is a voter saying by giving a candidate a grade below the Approval Cutoff?

One could consider the LPG to be like Gerald Ford. Anybody better would make a good president, and anybody worse would be bad.

Grading candidate X below the LPG gives the voter a chance to say "I don't want X to win, but of all the alternatives, X would make fewest changes in the wrong direction. I also won't give X a passing grade because I want X to have as small a mandate as possible." This allows the losing minority to have some say in the outcome of the election, instead of leaving the choice to the strongest core support within the majority faction.

Tallying Votes

Rankings are added into a pairwise array. The approval scores of each candidate can be stored the diagonal cells, which are unused in other non-Approval-Condorcet hybrids.

To determine the winner:

  1. Eliminate any candidate that is defeated in a one-to-one match with any other higher-approved candidate. Such a defeat is called an Approval-consistent or definitive defeat.
  2. If more than one candidate remains, the winner is the one that defeats all other non-definitively-defeated candidates in one-to-one (pairwise) contests.

Handling Ties and Near Ties

In ordinary DMC, the winner is the candidate in Forest Simmon's P set, the set of candidates which are not approval-consistently defeated.

But in the event of a tie or near tie (say, margin within 0.01%), there may be no clear winner.

In that case, form the superset P*, the union of all sets P that result from all possible combinations of reversed ties or near-ties. Then choose the winner from P* using Random Ballot Order.