[EM] Instant Consensus voting method, by Steve Eppley

[Donald E Davison]

CC: cvdusa at aol.com
Comments: Authenticated sender is <seppley at alumni.caltech.edu>
Date: Tue, 1 Apr 1997 16:10:34 -0800
From: "Steve Eppley" <seppley at alumni.caltech.edu>
Subject: [ER] Instant Consensus voting method
To: elections-reform at igc.apc.org, election-methods-list at eskimo.com
Sender: owner-elections-reform at igc.apc.org
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Reply-To: elections-reform at igc.apc.org

Happy April,

This seems like a good time to explain the ideal single-winner
voting method, known as INSTANT CONSENSUS.  It's nearly
identical to the Instant Runoff method, being an iterative
elimination & transfer method, except that Instant Consensus
iterates completely instead of stopping short.

The Voting
----------
Each voter ranks the candidates in order of preference, from
most preferred to least preferred.

The Tally
---------
For each candidate,
   Count the ballots which rank that candidate most preferred.

Repeat the following...

   Eliminate the candidate which has the smallest count.

   For each ballot which was counted for the eliminated candidate,
      Transfer the ballot to the most preferred
      uneliminated candidate on that ballot.

...until all but one candidate has been eliminated.

Readers familiar with the Instant Runoff method--also known as
MPV or single-winner STV, and sometimes referred to using the
generic name Preference Voting--will see at a glance that
Instant Consensus is nearly identical, except that it iterates
one extra round in order to find the candidate which achieves
*true unanimity*, rather than settling for Instant Runoff's
meager "true majority."

---Steve     (Steve Eppley    seppley at alumni.caltech.edu)