Plurality criterion

Statement of Criterion

If the number of ballots ranking A as the first preference is greater than the number of ballots on which another candidate B is given any preference, then B must not be elected.

Complying Methods

First-Preference Plurality, Approval voting, IRV, and many Condorcet methods (using winning votes as defeat strength) satisfy the Plurality criterion. Condorcet methods using margins as the measure of defeat strength fail it, as does Raynaud (using either winning votes or margins as the measure of defeat strength), and also Minmax(pairwise opposition).

Example where Score voting fails if the definition of the criterion is extended to scored ballots:

3 A:1

1 C:5 B:4

1 D:5 B:4

Scores are A 3, C 5, B 8, D 5, making B the winner. Yet when looking at the rankings:

3 A

1 C>B

1 D>B

B is preferred on 2 ballots, while A is preferred 1st on 3 ballots.

Commentary

When the Plurality criterion requires that B not be elected, it means that even if all the voters who gave B some ranking were to elevate him to the top position, he would still not be the First Preference Plurality winner.

It also means that A has a stronger pairwise victory over B than B has even a path of victories to any other candidate.

It is conceivable that if B were elected, voters might not consider this a legitimate result.