Majority Choice Approval

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Revision as of 23:38, 18 October 2010 by imported>Homunq (→‎Criteria compliance)
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Majority Choice Approval (MCA) is a class of rated voting systems which attempt to find majority support for some candidate. It is closely related to Bucklin Voting, which refers to ranked systems using similar rules. In fact, some people consider MCA a subclass of Bucklin, and call it things like ER-Bucklin (for Equal-Ratings-[allowed] Bucklin).

How does it work?

Voters rate candidates into a fixed number of rating classes. There are commonly 3, 4, 5, or even 100 possible rating levels. The following discussion assumes 3 ratings, called "preferred", "approved", and "unapproved".

If one and only one candidate is preferred by an absolute majority of voters, that candidate wins. If not, the same happens if there is only one candidate approved by a majority.

If the election is still unresolved, one of two things must be true. Either multiple candidates attain a majority at the same rating level, or there are no candidates with an absolute majority at any level. In either case, there are different ways to resolve between the possible winners - that is, in the former case, between those candidates with a majority, or in the latter case, between all candidates.

The possible resolution methods include:

  • MCA-A: Most approved candidate (most votes above lowest possible rating)
  • MCA-P: Most preferred candidate (most votes at highest possible rating)
  • MCA-M: Candidate with the highest score at the rating level where an absolute majority first appears, or MCA-A if there are no majorities.
  • MCA-S: Range or Score winner, using (in the case of 3 ranking levels) 2 points for preference and 1 point for approval.
  • MCA-R: Runoff - Two "finalists" are chosen by one or two methods, such as one of the methods above or a equality-allowed Condorcet method over the given ballots. The finalists are then measured against each other using one of the following methods:
    • MCA-IR: Instant runoff (Condorcet-style, using ballots): Ballots are recounted for whichever one of the finalists they rate higher. Ballots which rate both candidates at the same level are counted for neither.
    • MCA-AR: Actual runoff: Voters return to the polls to choose one of the finalists. This has the advantage that one candidate is guaranteed to receive the absolute majority of the valid votes in the last round of voting of the system as a whole.

A note on the term MCA

"Majority Choice Approval" was first used to refer to a specific form, which would be 3-level MCA-AR in the nomenclature above. Later, a voting system naming poll chose it as a more-accessible replacement term for ER-Bucklin.

Criteria compliance

All MCA variants satisfy the Plurality criterion, the Majority criterion for solid coalitions, Monotonicity (for MCA-AR, assuming first- and second- round votes are consistent), and Minimal Defense (which implies satisfaction of the Strong Defensive Strategy criterion).

All of the methods are matrix-summable for counting at the precinct level. Only MCA-IR actually requires a matrix (or, possibly two counting rounds), and is thus "summable for k=2" ; the others require only O(N) tallies, and are thus "summable for k=1".

The Participation criterion and its stronger cousin the Consistency criterion, as well as the Later-no-harm criterion are not satisfied by any MCA variant, although MCA-P only fails Participation if the additional vote causes an approval majority.

Other criteria are satisfied by some, but not all, MCA variants. To wit:

  • Clone Independence is satisfied by most MCA versions. In fact, even the stronger Independence of irrelevant alternatives is satisfied by MCA-A, MCA-P, MCA-M, and MCA-S. Clone independence is satisfied along with the weaker and related ISDA by MCA-IR and MCA-AR, if ISDA-compliant Condorcet methods (ie, Schulze) are used to choose the two "finalists". Using simpler methods (such as MCA itself) to decide the finalists, MCA-IR and MCA-AR are not strictly clone independent.
  • The Condorcet criterion is satisfied by MCA-IR if the pairwise champion (aka CW) is visible on the ballots and would beat at least one other candidate by an absolute majority. It is satisfied by MCA-AR if at least half the voters at least approve the PC in the first round of voting. These methods also satisfy the Strategy-Free criterion if an SFC-compliant method such as Schulze is used to pick at least one of the finalists. All other MCA versions, however, fail the Condorcet and strategy-free criteria.

Thus, the method which satisfies the most criteria is MCA-AR, using Schulze over the ballots to select one finalist and MCA-P to select the other. Also notable are MCA-M and MCA-P, which, as rated methods (and thus ones which fail Arrow's ranking-based Universality criterion), are able to seem to "violate Arrow's Theorem" by simultaneously satisfying monotonicity and independence of irrelevant alternatives (as well as of course sovereignty and non-dictatorship).

An example

Tennessee's four cities are spread throughout the state

Imagine that Tennessee is having an election on the location of its capital. The population of Tennessee is concentrated around its four major cities, which are spread throughout the state. For this example, suppose that the entire electorate lives in these four cities, and that everyone wants to live as near the capital as possible.

The candidates for the capital are:

  • Memphis, the state's largest city, with 42% of the voters, but located far from the other cities
  • Nashville, with 26% of the voters, near the center of Tennessee
  • Knoxville, with 17% of the voters
  • Chattanooga, with 15% of the voters

The preferences of the voters would be divided like this:

42% of voters
(close to Memphis)
26% of voters
(close to Nashville)
15% of voters
(close to Chattanooga)
17% of voters
(close to Knoxville)
  1. Memphis
  2. Nashville
  3. Chattanooga
  4. Knoxville
  1. Nashville
  2. Chattanooga
  3. Knoxville
  4. Memphis
  1. Chattanooga
  2. Knoxville
  3. Nashville
  4. Memphis
  1. Knoxville
  2. Chattanooga
  3. Nashville
  4. Memphis

Assume half of voters in each city rate one city preferred, two cities approved, and one city unapproved; and half rate one preferred, one approved, and two unapproved.

City Preferred Approved
Memphis 42 42
Nashville 26 84
Chatanooga 15 79
Knoxville 17 45

There is no preferred majority winner. Therefore approved votes are added. This moves Nashville and Chatanooga above 50%, so a winner can be determined. All the given resolution methods would pick Nashville.

Various strategy attempts are possible in this scenario, but all would likely fail. If the eastern and western halves of the state both strategically refused to approve each other, in an attempt by the eastern half to pick Chatanooga, Nashville would still win. If Memphis, Nashville, and Chatanooga all bullet-voted in the hopes of winning, most Knoxville voters would probably extend approval as far as Nashville to prevent a win by Memphis, and/or at least a few Memphis voters (>8% overall, out of 42%) would approve Nashville to stop Chatanooga from winning. Either one of these secondary groups would be enough to ensure a Nashville win in any of the MCA variants.