Distributed Score Voting
Distributed Score Voting (DSV) is a Single-Winner and Multi-Winner Cardinal voting system.
Procedure
Voting
Each voter has 100 points to distribute among the candidates according to his preferences.
All candidates in the vote have 0 points by default.
Counting the votes
W = sum of all the points in the original vote (100 for all voters, at the beginning).
1) All head-to-head matches are conducted between candidates. In head-to-head, the candidate who has more points in a vote than his opponent receives W points from the vote. The candidate who gets the most points wins the head-to-head.
Graphically, each candidate is a node; the head-to-head is represented by an arrow, leaving the winning candidate, entering the losing candidate. The tie is represented as a double arrow entering, that is both candidates are considered losers.
2) Find the smallest set X (Smith set) of nodes that don’t have incoming arrows, coming from outside the set.
Then remove all candidates not in X from the votes.
3) Convert the votes into a range form, assigning 0 points to the candidates with the lowest score and normalizing* the remaining candidates, using the following formula:
M = candidate with the highest score, before normalization.
v0 = current value of candidate C, to be normalized.
v1 = value of candidate C, after normalization.
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4) Add up the points for each candidate of the range votes, and the candidate who has the highest sum, wins.
The choice of the single winner ends here.
5) If you want to have more winners, then remove the single-winner from all original votes, repeating the whole procedure from point 1.
The value W of each original vote is reduced by the points assigned to the removed candidate.
By repeating this process several times, you can get as many winners as you like, which will be those removed in point 5.
6) If you want to know the % of victory of the winning candidates then, in each original vote, you must remove all the candidates who haven’t won, and normalize* the vote with the formula used in point 3 (with W=100 fixed). The sum of points for each candidate will indicate the % of victory.
Head-to-head
In a head-to-head between candidates A and B, a vote like A[10], B[30], C[60], D[0] could be treated in 2 different forms:
1) A[25], B[75] or A[33] B[100]
This form is subject to some problems:
- in a context with only one winner and two candidates, the voter is unlikely to want to distribute his points in that way.
- greatly increase the tactical vote in which voters accumulate points on their preferred candidate.
- prevent the DSV to meet the following criteria: majority criterion, majority loser criterion, mutual majority criterion.
2) A[0], B[100] that is, 0 to the minor and maximum to the major
This form avoids all the problems mentioned above.
Simplified vote writing
To make the writing of the vote more comprehensible and simple, the voter can be left with almost complete freedom in the use of numerical values or only X.
Before the counting process, the votes will be normalized to 100-point votes, where the Xs are considered as equal weight values.
Examples of how a vote can be written by the voter and subsequently, in the counting, converted into 100 points:
X,0,0,0,0 → 100,0,0,0,0
X,X,X,X,0 → 25,25,25,25,0
4,3,2,1,0 → 40,30,20,10,0
40,6,3,1,0 → 80,12,6,2,0
101,0,0,0,0 → 100,0,0,0,0
The complexity in writing the vote adapts to the voter, and it’s also noted that, if 101 or 99 points are mistakenly distributed, the vote will still be valid.
Criteria
Criteria met by DSV:
- Majority criterion
- Majority loser criterion
- Mutual majority criterion
- Condorcet criterion
- Condorcet loser criterion
- Smith criterion
- Independence of irrelevant alternatives
- Independence of clones criterion
- Monotonicity criterion
- Reversal symmetry
- Pareto criterion
Criteria not met by DSV:
- Participation criterion
- Consistency criterion
- Later-no-harm criterion
- Later-no-help criterion
- Favorite betrayal criterion
The first two criteria not met are derived mainly from the fact that DSV wants to ensure the victory of the candidate who wins all the head-to-head (when it exists).
The last 3 unmet criteria can instead generate tactical votes, described below.
Tactical votes
In an election, the results of the head-to-head are the following: A>B , B>C , C>D , D>A , A>C , D>B and in the end wins B.
A voter who in this case supported the candidates as follows: A>D>B>C he could change his vote as follows: A>D>C>B to favor C more than B (without disadvantaging A and D).
This tactical vote could cause B to lose head-to-head between B and C and in this case B would be the candidate who loses all head-to-head, being eliminated immediately. The winner would no longer be B.
This type of tactical vote works only if:
- there is a condorcet paradox which includes at least 4 candidates.
- through the tactical vote, the candidate who should have been the winner can be taken out of the Smith set.
- the new winner is actually a better candidate than the previous one (the new winner in the example could also be C).
- the voter has a fairly precise knowledge of the likely ballots result, without which this tactical vote would turn against him.