Difference between revisions of "Prefer Accept Reject voting"
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* It fails [[Independence of irrelevant alternatives]], but passes [[Local independence of irrelevant alternatives]].  * It fails [[Independence of irrelevant alternatives]], but passes [[Local independence of irrelevant alternatives]].  
−  * It fails the [[Condorcet criterion]], but for any set of voters such that an honest majority Condorcet winner exists, there always exists a strong equilibrium set of strictly semihonest ballots that elects that CW.  +  * It fails the [[Condorcet criterion]], but for any set of voters such that an honest majority Condorcet winner exists, there always exists a strong equilibrium set of strictly semihonest ballots that elects that CW. (Note that this is not true for any strictlyranked Condorcet system!) 
* It fails the [[participation criterion]] but passes the [[semihonest participation criterion]].  * It fails the [[participation criterion]] but passes the [[semihonest participation criterion]]. 
Revision as of 03:36, 11 November 2016
Prefer Accept Reject (PAR) voting works as follows:
 Voters can Prefer, Accept, or Reject each candidate. Default is "Reject" for voters who do not explicitly reject any candidates, and "Accept" otherwise.
 Candidates with a majority of Reject, or with under 25% Prefer, are disqualified, unless that would disqualify all candidates.
 Each voter gives 1 point to each noneliminated candidate they prefer; and any voter who gave no such points (because their preferred candidates were all eliminated) gives 1 point to each noneliminated candidate they accept. The winner is the candidate with the most points.
Contents
Relationship to NOTA
If all the candidates in the first round got a majority of reject, then the voters have sent a message that none of the candidates are good, akin to a result of "none of the above" (NOTA). PAR still gives a winner, but it might be good to have a rule that such a winner could only serve one term, or perhaps a softer rule that if they run for the same office again, the information of what percent of voters rejected should be next to their name on the ballot.
Criteria compliance
PAR voting passes the majority criterion, the mutual majority criterion, Local independence of irrelevant alternatives (under the assumption of fixed "honest" ratings for each voter for each candidate), Independence of clone alternatives, Monotonicity, polytime, resolvability.
There are a few criteria for which it does not pass as such, but where it passes related but weaker criteria. These include:
 It fails Independence of irrelevant alternatives, but passes Local independence of irrelevant alternatives.
 It fails the Condorcet criterion, but for any set of voters such that an honest majority Condorcet winner exists, there always exists a strong equilibrium set of strictly semihonest ballots that elects that CW. (Note that this is not true for any strictlyranked Condorcet system!)
 It fails the participation criterion but passes the semihonest participation criterion.
 It fails O(N) summability, but can get that summability with twopass tallying (first determine who's eliminated, then retally).
 It may pass the majority Condorcet loser criterion (?).
 It fails the laternohelp criterion, but passes if there is at least one candidate above the elimination thresholds (which is always true, for instance, if there are some three candidates who get 3 different ratings on every ballot).
It fails the consistency criterion, the Condorcet loser criterion, reversibility, the majority loser criterion, the Strategyfree criterion, and the laternoharm and laternohelp criteria.
Favorite betrayal?
PAR voting fails the favorite betrayal criterion (FBC). For instance, consider the following "nondisqualifying centersqueeze" scenario:
 35: AX>B
 10: B>A
 10: B>AC
 5: B>C
 40: C>B
None are eliminated, so C wins with 40 points (against 35, 25, 35 for A, B, and X). However, if 6 of the first group of voters strategically betrayed their true favorite A, the situation would be as follows:
 29: AX>B
 6: X>B
 10: B>A
 10: B>AC
 5: B>C
 40: C>B
Now, A is eliminated with 51% rejection; so B (the CW) wins.
However, there are several ways to "rescue" FBClike behavior for this system.
First, add a "compromise" option to the ballot, as described in FBPPAR.
Second, restrict the domain to voting scenarios which meet the following restrictions:
 Each candidate either comes from one of no more than 3 "ideological categories", or is "nonviable".
 No "nonviable" candidate is preferred by more than 25%.
 Each voter rejects at least one of the 3 "ideological categories" (that is, rejects all candidates in that category).
If the above restrictions hold, then PAR voting would meet FBC. It is arguably likely that realworld voting scenarios will meet the above restrictions, except for a negligible fraction of "ideologically atypical" voters. For instance, in the first scenario above, the B>AC voters would probably actually vote either B>A or B>C.
Third, note that in any scenario where it fails that for some small group, there is a rational strategy for some superset of that group which does not involve betrayal. For instance, in first scenario above, if 11 of the AX>B voters switch to >AXB, then A is eliminated without any betrayal.
An example
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 on Wikipedia, the state's largest city, with 42% of the voters, but located far from the other cities
 Nashville on Wikipedia, with 26% of the voters, near the center of Tennessee
 Knoxville on Wikipedia, with 17% of the voters
 Chattanooga on Wikipedia, 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) 





Assume voters in each city preferred their own city; rejected any city that is over 200 miles away or is the farthest city; and accepted the rest.
City  P  A  R  tally 

Memphis  42  0  58  NA 
Nashville  26  74  0  100 
Chattanooga  15  43  42  NA 
Knoxville  17  41  42  NA 
Memphis is rejected by a majority, and is eliminated. Chattanooga and Knoxville both get less than 25% preference, so they are also eliminated. Nashville wins with a tally of 100%. This is a strong equilibrium; no rational strategy from any faction or combination thereof would change the winner. Knoxville and/or Chattanooga could each prevent the other from being eliminated, but Nashville would still win with a tally of at least 68 (the ballots of Nashville and Memphis).
(If Memphis voters rejected Nashville, then Chattanooga or Knoxville could win by conspiring to reject Nashville and accept Memphis. However, Nashville could stop this by rejecting them. Thus this strategy would not work without extreme foolishness from both Memphis and Nashville voters, and extreme amounts of strategy from the others.)
Discussion
Logic for 25%preferred threshold (step 2)
The 25%preferred threshold in step 2 is exactly enough so that, in a 3candidate election where all voters give all three grades, there will always be at least 1 candidate who passes the thresholds to not be disqualified. This does not hold for an election with 4 or more candidates; but even in those cases, it is usually reasonable to hope that the top 3 candidates combined will get enough preferences to ensure that at least one of them is above the 25% threshold.