Social Choice

Python program to calculate the winner of well-known winner selection methods, including the plurality method, the runoff method, the elimination method, the Borda count, and the pairwise-comparison method (a.k.a. Condorcet mthod).

Usage

1. Prepare a csv file which represents voters' preference schedule.

• The first line of the csv file consists of candidates. Any string can be used to represent a candidate.
• The rest of the file consists of voters' preferences. The candidate put at the first column is interpreted as the most preffered candidate.
• For example, if there are 4 candidates and 18 voters, the csv file should be like this:

A,B,C,D
A,B,C,D
D,C,B,A
B,C,A,D
A,B,C,D
A,B,C,D
A,B,C,D
B,C,A,D
A,B,C,D
B,C,A,D
B,C,A,D
B,C,A,D
C,B,A,D
C,B,A,D
C,B,A,D
D,C,B,A
C,B,A,D
D,C,B,A
A,B,C,D

2. Run sc.py at the command line with the csv file as an argument:

$ python3 sc.py preferences.csv

• You can use the -m or --method optional argument to specify a winner-selection method like this:

$ python3 sc.py preferences.csv -m borda

• The possible options are
  • the plurality method (plurality),
  • the runoff method (runoff),
  • the elimination method (elimination),
  • the Borda count (borda),
  • and the pairwise comparison method a.k.a. Condorcet method (pairwise).

3. The result will be displayed on the screen like this:

$ python3 sc.py preferences.csv -m borda
Borda count

Preference Schedule:
Voter 1: A, B, C, D
Voter 2: D, C, B, A
Voter 3: B, C, A, D
Voter 4: A, B, C, D
Voter 5: A, B, C, D
Voter 6: A, B, C, D
Voter 7: B, C, A, D
Voter 8: A, B, C, D
Voter 9: B, C, A, D
Voter 10: B, C, A, D
Voter 11: B, C, A, D
Voter 12: C, B, A, D
Voter 13: C, B, A, D
Voter 14: C, B, A, D
Voter 15: D, C, B, A
Voter 16: C, B, A, D
Voter 17: D, C, B, A
Voter 18: A, B, C, D

Detailed Preference Schedule:
5 Voters: B, C, A, D
4 Voters: C, B, A, D
3 Voters: D, C, B, A
6 Voters: A, B, C, D

Borda scores: {'C': 52, 'A': 45, 'B': 56, 'D': 27}
The winner(s) is(are) ['B']