Proportional Ordering

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Proportional Ordering is a ranking method proposed by James Green-Armytage in December 2004. The ordering is intended so that a semi-proportional set of n candidates can be obtained by taking the first n candidates from the ranking.

Definition[edit | edit source]

Start by choosing the Condorcet winner. To choose the kth candidate, perform a CPO-STV tally considering only the outcomes that include the previously chosen k-1 candidates; this will add one new candidate to the ranking. Repeat the process until all candidates have been ranked.

Example[edit | edit source]

Suppose there are four candidates: Andrea (A), Brad (B), Carter (C), and Delilah (D). The ballots are:

  • 5: A>B>C>D
  • 17: A>C>B>D
  • 8: D

First Candidate[edit | edit source]

Andrea is the Condorcet winner, so she is the first candidate in the ordering.

Second Candidate[edit | edit source]

To choose the second-place candidate, we consider the sets of two candidates including Andrea: {A, B}, {A, C}, and {A, D}.

In the CPO-STV comparison for {A, B} versus {A, C}, D is eliminated, leaving

  • 5: A>B>C
  • 17: A>C>B

After transferring A's excess votes, {A, C} beats {A, B}.

The CPO-STV comparison for {A, B} versus {A, D} eliminates C, leaving

  • 22: A>B>D
  • 8: D

With the two-seat Droop quota of 11 votes, A meets the quota with 11 excess votes, all transferring to B, giving a vote count of:

  • A: 11
  • B: 11
  • D: 8
  • A+B: 22
  • A+D: 19

So {A, B} beats {A, D}.

Finally, the CPO-STV comparison for {A, C} versus {A, D} gives a vote count of:

  • A: 11
  • C: 11
  • D: 8
  • A+C: 22
  • A+D: 19

So {A, C} beats {A, D}.

The set {A, C} is the Condorcet winner (beating both {A, B} and {A, D}). Therefore, Carter is the second candidate in the proportional ordering.

Third candidate[edit | edit source]

We now consider the sets of three candidates containing both Andrea and Carter, i.e., {A, C, B} and {A, C, D}. The inital first-choice vote counts are:

  • A: 22
  • B: 8

Andrea is in both outcomes, so her votes are transferred. Using the three-seat Droop quota of 8 votes with fractional transfer gives her a retention fraction of 1-8/22 = 7/11. Applying this to the A>B>C>D and A>C>B>D ballots gives.

  • 3+2/11: (A)>B>C>D
  • 10+9/11: (A)>C>B>D
  • 8: D

Now, Carter has a surplus as well, all of which transfers to Brad, giving the ballots:

  • 3+2/11: (A)>B>(C)>D
  • 2+9/11: (A>C)>B>D
  • 8: D

Vote transfers are now complete. The vote totals for each candidate are:

  • A: 8 (quota)
  • B: 6
  • C: 8 (quota)
  • D: 8
  • A+C+B: 22
  • A+C+D: 24

Thus, {A, C, D} beats {A, C, B}, so Delilah is the next candidate chosen.

Final ordering[edit | edit source]

Brad is now the only candidate left and is added to the end of the ranking. The final proportional ordering is:

  1. Andrea
  2. Carter
  3. Delilah
  4. Brad

See also[edit | edit source]