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Copyright 2013, 2010, 2007, Pearson, Education, Inc. Section 15.2 Flaws of Voting

Copyright 2013, 2010, 2007, Pearson, Education, Inc. Section 15.2 Flaws of Voting

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Copyright 2013, 2010, 2007, Pearson, Education, Inc. Fairness Criteria Mathematicians and political scientists have agreed that a voting method should meet the following four criteria in order for the voting method to be considered fair. Majority Criterion Head-to-head Criterion Monotonicity Criterion Irrelevant Alternatives Criterion

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Page 1: Copyright 2013, 2010, 2007, Pearson, Education, Inc. Section 15.2 Flaws of Voting

Copyright 2013, 2010, 2007, Pearson, Education, Inc.

Section 15.2

Flaws of Voting

Page 2: Copyright 2013, 2010, 2007, Pearson, Education, Inc. Section 15.2 Flaws of Voting

Copyright 2013, 2010, 2007, Pearson, Education, Inc.

What You Will Learn

Fairness CriteriaMajority CriterionHead-to-Head CriterionMonotonicity CriterionIrrelevant Alternative Criterion

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Page 3: Copyright 2013, 2010, 2007, Pearson, Education, Inc. Section 15.2 Flaws of Voting

Copyright 2013, 2010, 2007, Pearson, Education, Inc.

Fairness CriteriaMathematicians and political scientists have agreed that a voting method should meet the following four criteria in order for the voting method to be considered fair.Majority CriterionHead-to-head CriterionMonotonicity CriterionIrrelevant Alternatives Criterion

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Page 4: Copyright 2013, 2010, 2007, Pearson, Education, Inc. Section 15.2 Flaws of Voting

Copyright 2013, 2010, 2007, Pearson, Education, Inc.

Majority Criterion

If a candidate receives a majority (more than 50%) of the first-place votes, that candidate should be declared the winner.

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Page 5: Copyright 2013, 2010, 2007, Pearson, Education, Inc. Section 15.2 Flaws of Voting

Copyright 2013, 2010, 2007, Pearson, Education, Inc.

Head-to-Head Criterion

If a candidate is favored when compared head-to-head with every other candidate, that candidate should be declared the winner.

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Page 6: Copyright 2013, 2010, 2007, Pearson, Education, Inc. Section 15.2 Flaws of Voting

Copyright 2013, 2010, 2007, Pearson, Education, Inc.

Monotonicity Criterion

A candidate who wins a first election and then gains additional support without losing any of the original support should also win a second election.

15.2-6

Page 7: Copyright 2013, 2010, 2007, Pearson, Education, Inc. Section 15.2 Flaws of Voting

Copyright 2013, 2010, 2007, Pearson, Education, Inc.

Irrelevant Alternatives CriterionIf a candidate is declared the winner of an election and in a second election one or more of the other candidates is removed, the previous winner should still be declared the winner.

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Page 8: Copyright 2013, 2010, 2007, Pearson, Education, Inc. Section 15.2 Flaws of Voting

Copyright 2013, 2010, 2007, Pearson, Education, Inc.

Summary of the Voting Methods and Whether They Satisfy the Fairness Criteria

May not satisfy

May not satisfy

May not satisfy

May not satisfy

Irrelevant alternatives

May not satisfy

May not satisfy

May not satisfy

Always satisfies

Monotonicity

Always satisfies

May not satisfy

May not satisfy

May not satisfy

Head-to-head

Always satisfies

Always satisfies

May not satisfy

Always satisfies

Majority

Pairwise comparison

Plurality with elimination

Borda count

Plurality

Method

Criteria

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Page 9: Copyright 2013, 2010, 2007, Pearson, Education, Inc. Section 15.2 Flaws of Voting

Copyright 2013, 2010, 2007, Pearson, Education, Inc.

Arrow’s Impossibility TheoremIt is mathematically impossible for any democratic voting method to simultaneously satisfy each of the fairness criteria:• The majority criterion• The head-to-head criterion• The monotonicity criterion• The irrevelant alternative criterion

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