[EM] RCIPE version 2
VoteFair
electionmethods at votefair.org
Tue Jul 20 18:19:26 PDT 2021
Version 2 of the Ranked Choice Including Pairwise Elimination (RCIPE)
voting method is now coded.
Here's a link to the scatter plot that shows its Clone Independence and
IIA (Independence of Irrelevant Alternatives) success rates compared to
other single-winner methods:
https://www.rankedchoiceoregon.org/img/clone_iia_success_rates.jpg
Based on Kristofer's feedback on the scatter plot, I've moved the IRV
(instant-runoff voting) data points to the top to show that IRV has a
zero Clone Independence failure rate. (The CI measurements yield a
non-zero CI failure rate because IRV does not specify a tie breaker
method and there are lots of lower-level ties in these "stress test"
measurements.) I've removed the IRV-BTR data because it's so similar to
IRV.
Here is how RCIPE version 2 is counted:
* Eliminate the "pairwise losing candidate" (Condorcet loser) when there
is one.
* Calculate a "pairwise support count" for each candidate. Each ballot
contributes to that count the number of remaining(!) candidates who are
ranked lower than the specified candidate.
* Eliminate the candidate who has the smallest pairwise support count.
* Repeat the loop to eliminate the next candidate. The last candidate
remaining is the winner.
* If there is a tie for the smallest pairwise support count, use IRV
(instant-runoff voting) as the tie-breaker. Specifically, eliminate the
tied(!) candidate who is ranked highest -- compared to the other tied
candidates -- on the fewest ballots.
* If there is still a tie, eliminate the candidate who has the smallest
one-on-one pairwise count against any other tied candidate. (This
addition is based on recent suggestions here.)
That's it!
The change from RCIPE v1 to v2 consists of:
* Reversing the order of the "IRV" step and the tie-breaker step that
involves the pairwise support counts
* Ignoring the pairwise opposition counts
This RCIPE version 2 method "fails" lots of fairness criteria. However,
the success rates for the most important criteria are high, and similar
to -- and for CI even better than -- the Condorcet-Kemeny method. For
example, where C-K indicates the Condorcet-Kemeny method, and IPE
indicates Instant Pairwise Elimination:
Condorcet compliance
Method, 2 choices, 6 choices, 9 choices
C-K, 100, 100, 100
IPE, 100, 100, 100
RCIPEv2, 100, 100, 99.6
RCIPEv1, 100, 99.1, 96.5
Clone Independence
Method, 2 choices, 6 choices, 9 choices
C-K, 100, 94.2, 94.4
IPE, 100, 94.7, 93.7
RCIPEv2, 100, 98.4, 97.3
RCIPEv1, 100, 99.8, 99.6
Independence of Irrelevant Alternatives
Method, 6 choices, 9 choices
C-K, 83.7, 78.2
IPE, 83, 76.9
RCIPEv2, 78.5, 70.7
RCIPEv1, 77.5, 69.2
In other words, as is well-known, getting smaller failure rates for one
criterion requires accepting larger failure rates for other criteria.
This method is a nice compromise regarding failure rates.
The big advantages are:
* Simple for voters to understand how the counting is done. Whereas
STAR voting gets a candidate's score from the ballot's column heading,
RCIPE v2 gets a candidate's "score" from the ballot by counting how many
remaining candidates are ranked lower than that candidate.
* Pairwise losing candidates are identified using the same pairwise counts.
* Eliminates one candidate at a time, which many voters find to be
easier to understand and trust.
* Avoids using fractions or decimal numbers to allow two or more
candidates to be ranked at the same preference level, which was needed
for RCIPE v1.
* Is precinct summable!
As a bonus, the ballots can be counted manually. Or the calculations
can be done using a calculator and paper and pen.
Is there anything I'm overlooking? If not, I'll update the RCIPE
article on Electowiki. Of course I'll move the version 1 description to
the history section.
Thanks for any feedback.
Richard Fobes
The VoteFair guy
More information about the Election-Methods
mailing list