In voting systems, the minimax method is one of several Condorcet methods used for tabulating votes and determining a winner when using ranked voting in a single-winner election. It is also known as the Simpson–Kramer method, and the successive reversal method. Minimax selects as the winner the candidate whose greatest pairwise defeat is smaller than the greatest pairwise defeat of any other candidate.
Minimax selects the candidate for whom the greatest pairwise score for another candidate against him or her is the least such score among all candidates. Formally, let denote the pairwise score for against. Then the candidate, selected by minimax is given by:
Variants of the pairwise score
When it is permitted to rank candidates equally, or to not rank all the candidates, three interpretations of the rule are possible. When voters must rank all the candidates, all three variants are equivalent. Let be the number of voters ranking X over Y. The variants define the score for candidate X against Y as:
The number of voters ranking X above Y, but only when this score exceeds the number of voters ranking Y above X. If not, then the score for X against Y is zero. This variant is sometimes called winning votes.
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The number of voters ranking X above Y minus the number of voters ranking Y above X. This variant is called using margins.
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The number of voters ranking X above Y, regardless of whether more voters rank X above Y or vice versa. This variant is sometimes called pairwise opposition.
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When one of the first two variants is used, the method can be restated as: "Disregard the weakest pairwise defeat until one candidate is unbeaten." An "unbeaten" candidate possesses a maximum score against him which is zero or negative.
The results of the pairwise scores would be tabulated as follows:
indicates voters who preferred the candidate listed in the column caption to the candidate listed in the row caption
indicates voters who preferred the candidate listed in the row caption to the candidate listed in the column caption
Result: In all three alternatives Nashville, the capital in real life, has the lowest value and is elected winner.
Example with Condorcet winner that is not elected winner (for pairwise opposition)
Assume three candidates A, B and C and voters with the following preferences:
4% of voters
47% of voters
43% of voters
6% of voters
1. A and C
1. A
1. C
1. B
1. A and C
2. C
2. B
2. A and C
3. B
3. B
3. A
2. A and C
The results would be tabulated as follows:
indicates voters who preferred the candidate listed in the column caption to the candidate listed in the row caption
indicates voters who preferred the candidate listed in the row caption to the candidate listed in the column caption
Result: With the alternatives winning votes and margins, the Condorcet winner A is declared Minimax winner. However, using the pairwise opposition alternative, C is declared winner, since less voters strongly oppose him in his worst pairwise score against A than A is opposed by in his worst pairwise score against B.
Example without Condorcet winner
Assume four candidates A, B, C and D. Voters are allowed to not consider some candidates, so that their ballots are not taken into account for pairwise scores of that candidates. The results would be tabulated as follows:
indicates voters who preferred the candidate listed in the column caption to the candidate listed in the row caption
indicates voters who preferred the candidate listed in the row caption to the candidate listed in the column caption
Result: Each of the three alternatives gives another winner:
the winning votes alternative chooses A as winner, since it has the lowest value of 35 votes for the winner in his biggest defeat;
the margin alternative chooses B as winner, since it has the lowest difference of votes in his biggest defeat;
and pairwise opposition chooses the Condorcet loserD as winner, since it has the lowest votes of the biggest opponent in all pairwise scores.
