PSC-CLE

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PSC-CLE (Proportionality of Solid Coalitions by Condorcet Loser Elimination) is a preference voting system for multi-winner elections.

Voting[edit | edit source]

Each voter ranks all candidates in order of preference. For example:

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

Procedure[edit | edit source]

Step 1: Compute the elimination order.

The elimination order is determined using a Condorcet ranking method. The ranking is inverted so that the Condorcet Winner, if one exists, will be last in the elimination order.

Step 2: Compute the proportionality rules.

Each possible set of candidates is entitled to a number of seats equal to either the greatest integer less than (proportion of voters solidly committed to that set) × (seats + 1), or the number of candidates in that set, whichever is lower. ("Solidly committed" is defined as in Descending Solid Coalitions.)

Step 3: Using the elimination order found in Step 1, eliminate candidates one-by-one until the number of candidates remaining is equal to the number of seats. But if the elimination of a candidate would cause a proportionality rule to be violated, then do not eliminate that candidate.

Example[edit | edit source]

2 seats to be filled, 4 candidates: Andrea (A), Brad (B), Carter (C), and Delilah (D). The ballots are:

  • 15: A>B>C>D
  • 51: A>C>B>D
  • 4: D>A>B>C
  • 4: D>A>C>B
  • 4: D>B>A>C
  • 4: D>B>C>A
  • 4: D>C>A>B
  • 4: D>C>B>A

The pairwise victories are:

  • 78-12: A>B, A>C
  • 66-24: A>D, B>D, C>D
  • 63-27: C>B

The Condorcet ranking is A>C>B>D, so the elimination order is D,B,C,A.

The sets of candidates with solidly-committed voters are:

Set Solidly Committed Quotas Int. Quotas # Candidates Min. Seats
{A} 66/90 = 11/15 (11/15)×(2+1) = 2+1/5 2 1 1
{D} 24/90 = 4/15 (4/15)×(2+1) = 4/5 0 1 0
{A, B} 15/90 = 1/6 (1/6)×(2+1) = 1/2 0 2 0
{A, C} 51/90 = 17/30 17/30×(2+1) = 1+7/10 1 2 1
{A, D} 8/90 = 4/45 (4/45)×(2+1) = 4/15 0 2 0
{B, D} 8/90 = 4/45 (4/45)×(2+1) = 4/15 0 2 0
{C, D} 8/90 = 4/45 (4/45)×(2+1) = 4/15 0 2 0
{A, B, C} 66/90 = 11/15 (11/15)×(2+1) = 2+1/5 2 3 2
{A, B, D} 8/90 = 4/45 (4/45)×(2+1) = 4/15 0 3 0
{A, C, D} 8/90 = 4/45 (4/45)×(2+1) = 4/15 0 3 0
{B, C, D} 8/90 = 4/45 (4/45)×(2+1) = 4/15 0 3 0
{A, B, C, D} 90/90 = 1 1×(2+1) = 3 2 4 2

Thus, the proportionality rules are:

  • Elect A.
  • Elect at least 1 candidate from {A, C}.
  • Elect at least 2 candidates from {A, B, C}.
  • Elect at least 2 candidates from {A, B, C, D}.

or, equivalently:

  • Elect A.
  • Elect either B or C.

The first 2 candidates in the elimination order, D and B, can be removed without violating the proportionality rules. We're now left with the required 2 candidates, and so are finished.

The winners are Andrea and Carter.