User:Matijaskala/Probabilistic Approval Voting: Difference between revisions

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'''Probabilistic Approval Voting''' is a sequential [[Proportional representation|proportional voting system]] that uses either [[Approval ballot|approval]] or [[Score voting|score]] ballots. ItsProbabilistic winnerscalculations are foundused byas thea usetool to predict potential power balance of probabilisticelected calculationspolitical factions. The method itself is deterministic. The system is 2-level [[Summability criterion|precinct-summable]] and passes the [[universally liked candidate criterion]].
 
== Derivation ==
Given:
 
* <math display="inline">C</math> ... the set of all candidates
* <math display="inline">W</math> ... the set of already elected candidates
* <math display="inline">V(A)</math> ... number of voters who approve of A
* <math display="inline">V(A \and B)</math> ... number of voters who approve of both A and B
 
Let's say that there is a functioncandidate P:A C×Cproposes ->a [0,1]decision. which returns theThe probability that two given candidates belong to the same political faction. Then for a given candidate Asupports the expected number of elected candidates belonging to the same factiondecision equals <math display="inline">\sum_frac{XV(A \inand WB)} P{V(A,XB)}</math>. andExpected thenumber expectedof averagevotes loadthat ofthe A'sdecision votersgets equals <math display="inline">\frac{\sum_{XB \in W} P\frac{V(A,X \and B)}{V(AB)}</math>. If A is not already elected then we can calculate the expected average load of A's voters after A's election asor <math display="inline">\frac{1+\sum_{XB \in W} P\frac{V(A,X \and B)}{V(B)}}{V(A)}</math> per voter. Let'sTo callminimize number of votes per voter we maximize <math display="inline">\frac{V(A)}{1+\sum_{X \in W} P(A,X)}</math> A's score and <math display="inline">\frac{1+\sum_{XV(A \inand W}P(A,X)}{V(AX)}}</math> A's inverse score. If we keep electing candidates whose inverse score is at least as low as inverse hare quota then we can expect an outcome wherefor each winner'snewly voters'elected expected average load is below a certain limit aka a proportional outcomecandidate.
 
== Example ==
In practice, electing a candidate whose inverse score is at least as low as inverse hare quota will not always be possible. In that case we need to either add failsafe approvals which we previously didn't consider or elect the candidate with the lowest inverse score and hope it is low enough.
<blockquote>
 
29 AB
A reasonable choice for P(A,B) would be <math display="inline">\frac{V(A \and B)}{V(A \or B)}</math> which would give <math>\frac{V(A)}{1+\sum_{X \in W} \frac{V(A \and X)}{V(A \or X)}}</math> as the formula for the score of candidate A. In each step we elect the candidate with the highest score. This version of the system is 2-level precint-summable and passes universal liking condition.
 
1 B
Probabilistic voting can be done with score ballots. We start by treating maximum score as approval. Once every candidate's score falls below hare quota we progressively add lower scores.
 
14 C
 
</blockquote>
 
In each step we elect the candidate with the highest <math display="inline">\frac{V(A)}{1+\sum_{X \in W} \frac{V(A \and X)}{V(X)}}</math>.
 
<math display="inline">\frac{V(A \and B)}{V(B)} = \frac{29}{30}</math>
 
<math display="inline">\frac{V(A \and C)}{V(C)} = \frac{0}{14}</math>
 
<math display="inline">\frac{V(B \and C)}{V(B)} = \frac{0}{30}</math>
 
First seat:
 
A: <math display="inline">V(A)/1 = 29/1 = 29</math>
 
B: <math display="inline">V(B)/1 = 30/1 = 30</math>
 
C: <math display="inline">V(C)/1 = 14/1 = 14</math>
 
B is elected
 
Second seat:
 
A: <math display="inline">V(A)/(1 + \frac{V(A \and B)}{V(B)}) = 29/(1 + \frac{29}{30}) = 14,745762711864</math>
 
C: <math display="inline">V(C)/(1 + \frac{V(C \and B)}{V(B)}) = 14/(1 + \frac{0}{30}) = 14</math>
 
A is elected
 
== Score ballots ==
[[File:Probabilistic Approval Voting with score ballots.jpg|thumb|402x402px|One possible procedure to elect a candidate using score ballots]]
Probabilistic voting can be done with score ballots. We start by treating maximum score as approval. Once every candidate's score falls below hareHare quota we progressively add lower scores.
[[Category:Cardinal PR methods]]
[[Category:Probability]]
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