Proportional Subset Voting

Proportional Subset Voting (PSV) is a Single-Winner and Multi-Winner, Cardinal voting systems proposed by Aldo Tragni.

Procedure

Ballot use range [-MAX,MAX]. N is the number of winners.

For each vote, and for each subset of N candidates, the following procedure is applied, considering only the original ratings of the N candidates in the vote:

• the highest rating is divided by $\begin{equation}2^0\end{equation}$ , the 2nd highest rating is divided by $\begin{equation}2^1\end{equation}$ , ... , the N-th highest rating (which is the lowest) is divided by $\begin{equation}2^{N-1}\end{equation}$ .
• after this division, the ratings are added to obtain the value S.

By applying this procedure, in the end, we obtain for each vote a list of scores S, one for each subset.

The scores S, for each subset, are added together and the subset with the highest sum contains the N winners.

Example

The following example shows how scores S are obtained from a vote:

N = 2 (winners)
Original vote, with range [-4,4]:
A B[-4] C D
Subsets
A,B: 4/1 + -4/2 = 2
A,C: 4/1 + 0/2 = 4
A,D: 4/1 + 2/2 = 5
B,C: -4/1 + 0/2 = -4
B,D: -4/1 + 2/2 = -3
C,D: 0/1 + 2/2 = 1
Converted vote:
A,D A,C A,B C,D B,D[-3] B,C[-4]

The following example shows how the sums for each subset are obtained, given the converted votes:

A,D  A,C  A,B  C,D  B,D[-3]  B,C[-4]
A,D  A,C  A,B[-4] C,D  B,D[-3]  B,C
A,D  A,C  A,B  C,D  B,D[-4]  B,C[-3]
Sums for each subset:
A,D A,C A,B[-1] C,D B,D[-10] B,C[-6]
The winner is {A,C}

Subset Voting (category)

N is the number of winners.

Considering one vote, a score S is obtained for each subset of N candidates. This procedure is applied to all votes and returns converted votes.

Considering the subsets as single candidates, a Single-Winner system applicable on the converted votes, is used to obtain the winning subset.

The way of calculating S and the Single-Winner system chosen, determines the variant of Subset Voting.

Thiele method

Thiele method uses range [0,MAX] and divides the values by $\begin{equation}i\end{equation}$ with $\begin{equation}i=1,...,N\end{equation}$ .

PSV uses range [-MAX,MAX] and divides the values by $\begin{equation}2^i\end{equation}$ with $\begin{equation}i=0,...,N-1\end{equation}$ .