Nanson's methodElectoral methods
This series is part of the
Politicsand the Electionseries.
- Single member
- Plurality (first-past-the-post)
- Two-round system
- Exhaustive ballot
- Preferential systems
- Non-rank methods
- Proportional representation
- Semi-proportional representation
- Non-proportional multi-member
- Random selection
- 1 Nanson method
- 2 Baldwin method
- 3 Satisfied and failed criteria
- 4 Use of Nanson and Baldwin
- 5 Note
- 6 Advocacy
The Nanson method is based on the original work of the mathematician Edward J. Nanson.
Nanson's method eliminates those choices from a Borda count tally that are at or below the average Borda count score, then the ballots are retallied as if the remaining candidates were exclusively on the ballot. This process is repeated if necessary until a single winner remains.
This variant was devised by Joseph M. Baldwin and works like this:
Candidates are voted for on ranked ballots as in the Borda count. Then, the points are tallied in a series of rounds. In each round, the candidate with the fewest points is eliminated, and the points are re-tallied as if that candidate were not on the ballot.
Satisfied and failed criteria
The Nanson method and the Baldwin method satisfy the Condorcet criterion: since Borda always gives any existing Condorcet winner more than the average Borda points, the Condorcet winner will never be eliminated. They do not satisfy the independence of irrelevant alternatives criterion, the monotonicity criterion, the participation criterion, the consistency criterion and the independence of clones criterion, while they do satisfy the majority criterion, the mutual majority criterion, the Condorcet loser criterion, and the Smith criterion. The Nanson method satisfies reversal symmetry, while the Baldwin method violates reversal symmetry.
Use of Nanson and Baldwin
Nanson's method was used in city elections in the U.S. town of Marquette, Michigan in the 1920s. It was formally used by the Anglican Diocese of Melbourne and in the election of members of the University Council of the University of Adelaide. It was used by the University of Melbourne until 1983.
- A case for Baldwin's Method by Ian Fellows