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Module 1- Developmental Mathematics, Lecture notes of Mathematics

Notes from module 1 video lectures

Typology: Lecture notes

2020/2021

Uploaded on 12/09/2021

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1.3 The Plurality Method and Condorcet Criterion
Plurality Voting Method
In this method, the choice with the most first-preference votes is declared the
winner. Ties are possible, and would have to be settled through some sort of run-
off vote
This method is sometimes mistakenly called the majority method, or “majority
rules”, but it is not necessary for a choice to have gained a majority of votes to
win. A majority is over 50%. It is horrible for a winner to have a plurality without
having a majority.
Examples:
1. A survey asks to rank which West coast state people would prefer to live. The
results are below. Using the plurality method, select the winner.
C = California O = Oregon W = Washington
Number of
Voters
75 94 51 12 43 25
1st Choice C C O O W W
2nd Choice O W W C C O
3d Choice W O C W O C
Total votes: 300
C: 75 + 94 = 169
O: 51 + 12 = 63
W: 43 +25 = 68
California received the most first choice votes so they are the plurality winner
Since 160/300 = 56.39 so California would also be the majority winner
*remember- winner does not have to be the majority winner to be the
plurality winner*
2. A small group of college students rank the best destination for Spring Break.
Determine the winner using the plurality method.
S = San Diego L = Lake Havasu R = Rocky Point
Number of
Voters
4 4 2 5 2
1st Choice S S L R R
2nd Choice L R R S L
3rd Choice R L S L S
Total votes: 17
S: 4 + 4 = 8
L: 2
R: 5 + 2 = 7
San Diego received the most first choice votes so they are the plurality
winner.
Although they are the plurality winner, 8/17 = 47.17 which is not the majority
percent so they would not be the majority winner.
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1.3 The Plurality Method and Condorcet Criterion

Plurality Voting Method

• In this method, the choice with the most first-preference votes is declared the

winner. Ties are possible, and would have to be settled through some sort of run-

off vote

• This method is sometimes mistakenly called the majority method, or “majority

rules”, but it is not necessary for a choice to have gained a majority of votes to

win. A majority is over 50%. It is horrible for a winner to have a plurality without

having a majority.

Examples:

1. A survey asks to rank which West coast state people would prefer to live. The

results are below. Using the plurality method, select the winner.

C = California O = Oregon W = Washington

Number of Voters

1st Choice C C O O W W 2nd Choice O W W C C O 3d Choice W O C W O C

Total votes: 300

C: 75 + 94 = 169

O: 51 + 12 = 63

W: 43 +25 = 68

California received the most first choice votes so they are the plurality winner

Since 160/300 = 56.39 so California would also be the majority winner

*remember- winner does not have to be the majority winner to be the

plurality winner*

2. A small group of college students rank the best destination for Spring Break.

Determine the winner using the plurality method.

S = San Diego L = Lake Havasu R = Rocky Point

Number of Voters

1st Choice S S L R R 2nd Choice L R R S L 3rd Choice R L S L S

Total votes: 17

S: 4 + 4 = 8

L: 2

R: 5 + 2 = 7

San Diego received the most first choice votes so they are the plurality

winner.

Although they are the plurality winner, 8/17 = 47.17 which is not the majority

percent so they would not be the majority winner.

What’s wrong with plurality?

• If there are 3 or more choices, it is possible that a choice could lose but when

compared to a one-to-one comparison could be preferred over the plurality

winner. This violates a Fairness Criterion.

• The fairness criteria are statements that seem like they should be true in a fair

election.

Condorcet Criterion

• If there is a choice that is preferred in every one-to-one comparison with the

other choices, that choice should be the winner. We call this winner the

Condorcet Winner or Condorcet Candidate.

Number of Voters

1st Choice S S L R R 2nd Choice L R R S L 3rd Choice R L S L S

• Using the previous example regarding spring break destinations we will

determine the Condorcet WinnerTo find the Condorcet Winner we will do a

one-to-one comparison with our options

S vs L (San Diego is preferred over Lake Havasu) 13 to 4

S vs R (San Diego is preferred over Rocky Point) 8 to 9

L vs R (Lake Havasu is preferred over Rocky Point) 6 to 11

Rocky Point wins in a one-to-one comparison and is the Condorcet Winner

2. Find the Condorcet Candidate, if there is one.

Number of Voters 10 16 15 1st Choice B C A 2nd Choice C A C 3rd Choice A B B

Create a one-to-one comparison

A vs B : A is chosen over B by 15 + 16 voters while B is chosen over A by 10 = 31

to 10

A vs C : A is only chosen over C by 15 voters while C is chosen over A by 10 + 16 =

15 to 26

B vs C : B is only chosen over C by 10 voters while C is chosen over B by 16 + 15 =

10 to 31

Since C wins over both A and B in the one-to-one comparison then C is the

Condorcet Winner

*Notice the

Plurality

Winner

would be C

with a total

of 16 first