the method of pairwise comparisonsjlmartin/courses/math105-f11/lectures/chap… · the method of...
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The Method of Pairwise Comparisons
Suggestion from a Math 105 student (8/31/11): Hold aknockout tournament between candidates.
I This satisfies the Condorcet Criterion! A Condorcetcandidate will win all his/her matches, and therefore winthe tournament. (Better yet, seeding doesn’t matter!)
I But, if there is no Condorcet candidate, then it’s notclear what will happen.
I Using preference ballots, we can actually hold around-robin tournament instead of a knockout.
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The Method of Pairwise Comparisons (§1.5)
The Method of Pairwise Comparisons
Proposed by Marie Jean Antoine Nicolas de Caritat, marquisde Condorcet (1743–1794)
I Compare each two candidates head-to-head.
I Award each candidate one point for each head-to-headvictory.
I The candidate with the most points wins.
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The Method of Pairwise Comparisons
Number of Voters 14 10 8 4 11st choice A C D B C2nd choice B B C D D3rd choice C D B C B4th choice D A A A A
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The Method of Pairwise Comparisons
Number of Voters 14 10 8 4 11st choice A C D B C2nd choice B B C D D3rd choice C D B C B4th choice D A A A A
Compare A to B.
I 14 voters prefer A.
I 10+8+4+1 = 23 voters prefer B.
I B wins the pairwise comparison and gets 1 point.
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The Method of Pairwise Comparisons
Number of Voters 14 10 8 4 11st choice A C D B C2nd choice B B C D D3rd choice C D B C B4th choice D A A A A
Compare C to D:
I 14+10+1 = 25 voters prefer C.
I 8+4 = 12 voters prefer D.
I C wins the pairwise comparison and gets 1 point.
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The Method of Pairwise Comparisons
Number of Voters 14 10 8 4 11st choice A C D B C2nd choice B B C D D3rd choice C D B C B4th choice D A A A A
I Compare A to C. . . A to D. . . B to C. . . B to D. . .
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The Method of Pairwise Comparisons
Number of Voters 14 10 8 4 11st choice A C D B C2nd choice B B C D D3rd choice C D B C B4th choice D A A A A
A B C D Wins Losses PointsA
14 14 14 — B,C,D 0
B
23 18 28 A,C D 2
C
23 19 25 A,B,D — 3
Winner!
D
23 9 12 A B,C 1
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The Method of Pairwise Comparisons
Number of Voters 14 10 8 4 11st choice A C D B C2nd choice B B C D D3rd choice C D B C B4th choice D A A A A
A B C D Wins Losses PointsA 14
14 14 — B,C,D 0
B 23
18 28 A,C D 2
C
23 19 25 A,B,D — 3
Winner!
D
23 9 12 A B,C 1
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The Method of Pairwise Comparisons
Number of Voters 14 10 8 4 11st choice A C D B C2nd choice B B C D D3rd choice C D B C B4th choice D A A A A
A B C D Wins Losses PointsA 14 14 14
— B,C,D 0
B 23
18 28 A,C D 2
C 23
19 25 A,B,D — 3
Winner!
D 23
9 12 A B,C 1
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The Method of Pairwise Comparisons
Number of Voters 14 10 8 4 11st choice A C D B C2nd choice B B C D D3rd choice C D B C B4th choice D A A A A
A B C D Wins Losses PointsA 14 14 14
— B,C,D 0
B 23 18
28 A,C D 2
C 23 19
25 A,B,D — 3
Winner!
D 23
9 12 A B,C 1
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The Method of Pairwise Comparisons
Number of Voters 14 10 8 4 11st choice A C D B C2nd choice B B C D D3rd choice C D B C B4th choice D A A A A
A B C D Wins Losses PointsA 14 14 14
— B,C,D 0
B 23 18 28
A,C D 2
C 23 19 25
A,B,D — 3
Winner!
D 23 9 12
A B,C 1
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The Method of Pairwise Comparisons
Number of Voters 14 10 8 4 11st choice A C D B C2nd choice B B C D D3rd choice C D B C B4th choice D A A A A
A B C D Wins Losses PointsA 14 14 14 — B,C,D 0B 23 18 28 A,C D 2C 23 19 25 A,B,D — 3
Winner!
D 23 9 12 A B,C 1
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The Method of Pairwise Comparisons
Number of Voters 14 10 8 4 11st choice A C D B C2nd choice B B C D D3rd choice C D B C B4th choice D A A A A
A B C D Wins Losses PointsA 14 14 14 — B,C,D 0B 23 18 28 A,C D 2C 23 19 25 A,B,D — 3 Winner!D 23 9 12 A B,C 1
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Evaluating the Method of Pairwise Comparisons
I The Method of Pairwise Comparisons satisfies theMajority Criterion.
(A majority candidate will win every pairwise comparison.)
I The Method of Pairwise Comparisons satisfies theCondorcet Criterion.
(A Condorcet candidate will win every pairwisecomparison — that’s what a Condorcet candidate is!)
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Evaluating the Method of Pairwise Comparisons
I The Method of Pairwise Comparisons satisfies theMajority Criterion.
(A majority candidate will win every pairwise comparison.)
I The Method of Pairwise Comparisons satisfies theCondorcet Criterion.
(A Condorcet candidate will win every pairwisecomparison — that’s what a Condorcet candidate is!)
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Evaluating the Method of Pairwise Comparisons
I The Method of Pairwise Comparisons satisfies thePublic-Enemy Criterion.
(If there is a public enemy, s/he will lose every pairwisecomparison.)
I The Method of Pairwise Comparisons satisfies theMonotonicity Criterion.
(Ranking Candidate X higher can only help X in pairwisecomparisons.)
Does the Method of Pairwise Comparisons have anydrawbacks?
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Evaluating the Method of Pairwise Comparisons
I The Method of Pairwise Comparisons satisfies thePublic-Enemy Criterion.
(If there is a public enemy, s/he will lose every pairwisecomparison.)
I The Method of Pairwise Comparisons satisfies theMonotonicity Criterion.
(Ranking Candidate X higher can only help X in pairwisecomparisons.)
Does the Method of Pairwise Comparisons have anydrawbacks?
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Evaluating the Method of Pairwise Comparisons
I The Method of Pairwise Comparisons satisfies thePublic-Enemy Criterion.
(If there is a public enemy, s/he will lose every pairwisecomparison.)
I The Method of Pairwise Comparisons satisfies theMonotonicity Criterion.
(Ranking Candidate X higher can only help X in pairwisecomparisons.)
Does the Method of Pairwise Comparisons have anydrawbacks?
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How Many Pairwise Comparisons?
Problem #1: It’s somewhat inefficient. How many pairwisecomparisons are necessary if there are N candidates?How many spaces are there in the crosstable?
A B C D E
ABCDE
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How Many Pairwise Comparisons?
I N2 squares in crosstable
I N squares on the main diagonal don’t count
I Other squares all come in pairs
Number of comparisons =N2 − N
2=
N(N − 1)
2.
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Be Careful!
Number of pairwise comparisons with N candidates:
N(N − 1)
2.
Number of points on a Borda count ballot with N candidates:
N(N + 1)
2.
(To remember which is which, work out a small example, likeN = 3.)
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Evaluating the Method of Pairwise Comparisons
Problem #2 (the “rock-paper-scissors problem”):
Ties are very common under the Method of PairwiseComparisons.
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Evaluating the Method of Pairwise Comparisons
Number of voters 4 3 6
1st A B C2nd B C A3rd C A B
I The Method of Pairwise Comparisons results in athree-way tie.
I Under any other system we have discussed, C would win.
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Comparison of Voting Methods
Maj Cond PE Mono
Plurality Yes No No Yes
Borda Count No No Yes Yes
PWE Yes No Yes No
Pairwise Comparisons Yes Yes Yes Yes
Maj = Majority; Cond = Condorcet;PE = Public-Enemy; Mono = Monotonicity
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The IIA Criterion
Number of voters 9 11 7 6 3
1st A B D C D2nd C A B A C3rd D C C D B4th B D A B A
(1) Who wins?
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The IIA Criterion
Number of voters 9 11 7 6 3
1st A B D C D2nd C A B A C3rd D C C D B4th B D A B A
(2) What happens if D is disqualified?
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The IIA Criterion
Independence-Of-Irrelevant-Alternatives (IIA) Criterion:
If Candidate A is the winner of an election, and Candidate Bis suddenly disqualified, then A should still win the election.
We have just seen that the Method of Pairwise Comparisonsviolates IIA.
Unfortunately, none of the systems we have studied alwaysmeet the IIA Criterion!
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Comparison of Voting Methods
Maj Cond PE Mono IIA
Plurality Yes No No Yes No
Borda Count No No Yes Yes No
Plurality-With-Elim. Yes No Yes No No
Pairwise Comparisons Yes Yes Yes Yes No
Maj = Majority; Cond = Condorcet;PE = Public-Enemy; Mono = Monotonicity;IIA = Independence of Irrelevant Alternatives
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Which Voting System Is Best?
So, which voting system Is best?
There is no purely mathematical answer to this question.
Arrow’s Theorem: There is no voting system thatalways satisfies all four voting criteria – Majority,Condorcet, Monotonicity and IIA.
So, the answer depends which fairness criteria you think arethe most important.