Josh Engwer (TTU) Voting Methods 15 July / 49
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1 Voting Methods Contemporary Math Josh Engwer TTU 15 July 2015 Josh Engwer (TTU) Voting Methods 15 July / 49
2 Introduction In free societies, citizens vote for politicians whose values & opinions on contemporary issues align with theirs. Unfortunately, voting is not as clear-cut as one would expect. Consider U.S. presidential elections: 2000 Face-Off [ Al Gore (D) vs George W. Bush (R) ]: Gore received more (popular) votes than Bush, yet Bush won! Bush received more electoral votes and carried more states than Gore Florida (R) Primary [ Romney vs Gingrich vs Paul vs Santorum ]: Mitt Romney earned all 50 delegates yet he earned < 50% of the votes. Newt Gingrich earned no delegates yet he earned 32% of the votes. After a few primaries, most candidates drop out before most citizens have a chance to vote due to lack of funding. Winning depends not only on vote counts but also on how the votes are used! Because of these issues, several voting methods exist. Josh Engwer (TTU) Voting Methods 15 July / 49
3 Preference Ballots & Tables Suppose there are five candidates: A,B,C,D,E 1 st D 2 nd B Each voter uses a preference ballot to rank the candidates: 3 rd E 4 th A 5 th C Identical preference ballots are grouped together in a preference table: Preference st A E B A D 2 nd B D D C B 3 rd C A C E E 4 th D B E B A 5 th E C A D C Josh Engwer (TTU) Voting Methods 15 July / 49
4 Plurality Method (Definition) The simplest, most intuitive voting method is the Plurality Method: Definition (Plurality Method) SETUP: PROCESS: WINNER: Single-Winner Election has k candidates Each voter votes for one candidate Candidate receiving the most votes Plurality Method is typically used by: State elections Local elections City council elections School board elections Josh Engwer (TTU) Voting Methods 15 July / 49
5 Plurality Method (Example) WEX : Using Plurality Method, determine election winner: 1 st choice A D B C B 2 nd choice C B C D C 3 rd choice B A A A D 4 th choice D C D B A Josh Engwer (TTU) Voting Methods 15 July / 49
6 Plurality Method (Example) WEX : Using Plurality Method, determine election winner: 1 st choice A D B C B 2 nd choice C B C D C 3 rd choice B A A A D 4 th choice D C D B A With Plurality Method, only the first choice matters (in blue) Josh Engwer (TTU) Voting Methods 15 July / 49
7 Plurality Method (Example) WEX : Using Plurality Method, determine election winner: 1 st choice A D B C B 2 nd choice C B C D C 3 rd choice B A A A D 4 th choice D C D B A With Plurality Method, only the first choice matters (in blue) A has 10 votes B has = 9 votes C has 5 votes D has 7 votes Josh Engwer (TTU) Voting Methods 15 July / 49
8 Plurality Method (Example) WEX : Using Plurality Method, determine election winner: 1 st choice A D B C B 2 nd choice C B C D C 3 rd choice B A A A D 4 th choice D C D B A With Plurality Method, only the first choice matters (in blue) A has 10 votes B has = 9 votes C has 5 votes D has 7 votes Since A has the most votes, A is the winner Josh Engwer (TTU) Voting Methods 15 July / 49
9 Borda Count Method (Definition) Definition (Borda Count Method) SETUP: What if, instead, voters must rank each candidate? Single-Winner Election has k candidates PROCESS: (1) Each voter ranks all candidates as follows: The 1 st choice receives k points The 2 nd choice receives (k 1) points The 3 rd choice receives (k 2) points. The last choice receives 1 point (2) For each candidate, compute the total sum of points WINNER: Candidate receiving the most total points Borda Count Method is typically used in: sports polls, music industry awards, hiring CEO s,... Josh Engwer (TTU) Voting Methods 15 July / 49
10 Borda Count Method (Example) WEX : Using Borda Count Method, determine election winner: 1 st choice A D B C B 2 nd choice C B C D C 3 rd choice B A A A D 4 th choice D C D B A Josh Engwer (TTU) Voting Methods 15 July / 49
11 Borda Count Method (Example) WEX : Using Borda Count Method, determine election winner: 1 st choice/4 A D B C B 2 nd choice/3 C B C D C 3 rd choice/2 B A A A D 4 th choice/1 D C D B A Assign points to each preference catagory (in blue) Josh Engwer (TTU) Voting Methods 15 July / 49
12 Borda Count Method (Example) WEX : Using Borda Count Method, determine election winner: 1 st choice/4 A/(4 10) D/(4 7) B/(4 5) C/(4 5) B/(4 4) 2 nd choice/3 C/(3 10) B/(3 7) C/(3 5) D/(3 5) C/(3 4) 3 rd choice/2 B/(2 10) A/(2 7) A/(2 5) A/(2 5) D/(2 4) 4 th choice/1 D/(1 10) C/(1 7) D/(1 5) B/(1 5) A/(1 4) Assign points to each choice Josh Engwer (TTU) Voting Methods 15 July / 49
13 Borda Count Method (Example) WEX : Using Borda Count Method, determine election winner: 1 st choice/4 A/40 D/28 B/20 C/20 B/16 2 nd choice/3 C/30 B/21 C/15 D/15 C/12 3 rd choice/2 B/20 A/14 A/10 A/10 D/8 4 th choice/1 D/10 C/7 D/5 B/5 A/4 Assign points to each choice (and simplify) Josh Engwer (TTU) Voting Methods 15 July / 49
14 Borda Count Method (Example) WEX : Using Borda Count Method, determine election winner: 1 st choice/4 A/40 D/28 B/20 C/20 B/16 2 nd choice/3 C/30 B/21 C/15 D/15 C/12 3 rd choice/2 B/20 A/14 A/10 A/10 D/8 4 th choice/1 D/10 C/7 D/5 B/5 A/4 Tally points for each candidate: A: = 78 points Josh Engwer (TTU) Voting Methods 15 July / 49
15 Borda Count Method (Example) WEX : Using Borda Count Method, determine election winner: 1 st choice/4 A/40 D/28 B/20 C/20 B/16 2 nd choice/3 C/30 B/21 C/15 D/15 C/12 3 rd choice/2 B/20 A/14 A/10 A/10 D/8 4 th choice/1 D/10 C/7 D/5 B/5 A/4 Tally points for each candidate: A: = 78 points B : = 82 points Josh Engwer (TTU) Voting Methods 15 July / 49
16 Borda Count Method (Example) WEX : Using Borda Count Method, determine election winner: 1 st choice/4 A/40 D/28 B/20 C/20 B/16 2 nd choice/3 C/30 B/21 C/15 D/15 C/12 3 rd choice/2 B/20 A/14 A/10 A/10 D/8 4 th choice/1 D/10 C/7 D/5 B/5 A/4 Tally points for each candidate: A: = 78 points B: = 82 points C : = 84 points Josh Engwer (TTU) Voting Methods 15 July / 49
17 Borda Count Method (Example) WEX : Using Borda Count Method, determine election winner: 1 st choice/4 A/40 D/28 B/20 C/20 B/16 2 nd choice/3 C/30 B/21 C/15 D/15 C/12 3 rd choice/2 B/20 A/14 A/10 A/10 D/8 4 th choice/1 D/10 C/7 D/5 B/5 A/4 Tally points for each candidate: A: = 78 points B: = 82 points C: = 84 points D : = 52 points Josh Engwer (TTU) Voting Methods 15 July / 49
18 Borda Count Method (Example) WEX : Using Borda Count Method, determine election winner: 1 st choice/4 A/40 D/28 B/20 C/20 B/16 2 nd choice/3 C/30 B/21 C/15 D/15 C/12 3 rd choice/2 B/20 A/14 A/10 A/10 D/8 4 th choice/1 D/10 C/7 D/5 B/5 A/4 Tally points for each candidate: A: = 78 points B: = 82 points C: = 84 points D: = 52 points Since Candidate C has the most points, C is the winner Josh Engwer (TTU) Voting Methods 15 July / 49
19 Plurality-with-Elimination Method (Definition) Definition (Plurality-with-Elimination Method) SETUP: Single-Winner Election has k candidates PROCESS: (0) Compute total votes & # votes needed for a majority (1) If no candidate receives a majority of votes, then drop candidate(s) with fewest votes from the ballot (2) Conduct a new election round with updated ballot Assume voters don t change their preferences each round (3) Repeat (1)-(2) until a candidate receives a majority WINNER: Candidate receiving a majority of votes If voters prefer A to B and B to C, then if B s dropped, voters will prefer A to C. Plurality-with-Elimination Method is typically used in: Municipal Elections (e.g. city mayor) Josh Engwer (TTU) Voting Methods 15 July / 49
20 Plurality-with-Elimination Method (Example) WEX : Using Plurality-with-Elimination Mtd, determine election winner: 1 st choice A D B C B 2 nd choice C B C D C 3 rd choice B A A A D 4 th choice D C D B A Josh Engwer (TTU) Voting Methods 15 July / 49
21 Plurality-with-Elimination Method (Example) WEX : Using Plurality-with-Elimination Mtd, determine election winner: 1 st choice A D B C B 2 nd choice C B C D C 3 rd choice B A A A D 4 th choice D C D B A First of all, compute the total votes & votes needed for majority: Total votes = = 31 31/2 = 15.5 = At least 16 votes needed for a majority Josh Engwer (TTU) Voting Methods 15 July / 49
22 Plurality-with-Elimination Method (Example) WEX : Using Plurality-with-Elimination Mtd, determine election winner: 1 st choice A D B C B 2 nd choice C B C D C 3 rd choice B A A A D 4 th choice D C D B A Total votes = 31 At least 16 votes needed for a majority Round 1 Josh Engwer (TTU) Voting Methods 15 July / 49
23 Plurality-with-Elimination Method (Example) WEX : Using Plurality-with-Elimination Mtd, determine election winner: 1 st choice A D B C B 2 nd choice C B C D C 3 rd choice B A A A D 4 th choice D C D B A Total votes = 31 At least 16 votes needed for a majority Count 1 st -choice votes for each candidate: A = 10 < 16 = (NOT a majority) B = = 9 < 16 = (NOT a majority) C = 5 < 16 = (NOT a majority) D = 7 < 16 = (NOT a majority) Josh Engwer (TTU) Voting Methods 15 July / 49
24 Plurality-with-Elimination Method (Example) WEX : Using Plurality-with-Elimination Mtd, determine election winner: 1 st choice A D B C B 2 nd choice C B C D C 3 rd choice B A A A D 4 th choice D C D B A Total votes = 31 At least 16 votes needed for a majority Count 1 st -choice votes for each candidate: A = 10 < 16 = (NOT a majority) B = = 9 < 16 = (NOT a majority) C = 5 < 16 = (NOT a majority) D = 7 < 16 = (NOT a majority) Since no candidate has a majority, eliminate candidate(s) with fewest votes: C Josh Engwer (TTU) Voting Methods 15 July / 49
25 Plurality-with-Elimination Method (Example) WEX : Using Plurality-with-Elimination Mtd, determine election winner: 1 st choice A D B C B 2 nd choice C B C D C 3 rd choice B A A A D 4 th choice D C D B A Total votes = 31 At least 16 votes needed for a majority Since no candidate has a majority, eliminate candidate(s) with fewest votes: C Josh Engwer (TTU) Voting Methods 15 July / 49
26 Plurality-with-Elimination Method (Example) WEX : Using Plurality-with-Elimination Mtd, determine election winner: 1 st choice A D B C B 2 nd choice C B C D C 3 rd choice B A A A D 4 th choice D C D B A Total votes = 31 At least 16 votes needed for a majority Eliminate candidate C, moving every entry below C (in blue) up one row Josh Engwer (TTU) Voting Methods 15 July / 49
27 Plurality-with-Elimination Method (Example) WEX : Using Plurality-with-Elimination Mtd, determine election winner: 1 st choice A D B D B 2 nd choice B B A A D 3 rd choice D A D B A 4 th choice Total votes = 31 At least 16 votes needed for a majority Eliminate candidate C, moving every entry below C (in blue) up one row Josh Engwer (TTU) Voting Methods 15 July / 49
28 Plurality-with-Elimination Method (Example) WEX : Using Plurality-with-Elimination Mtd, determine election winner: 1 st choice A D B D B 2 nd choice B B A A D 3 rd choice D A D B A Total votes = 31 At least 16 votes needed for a majority Round 2 Josh Engwer (TTU) Voting Methods 15 July / 49
29 Plurality-with-Elimination Method (Example) WEX : Using Plurality-with-Elimination Mtd, determine election winner: 1 st choice A D B D B 2 nd choice B B A A D 3 rd choice D A D B A Total votes = 31 At least 16 votes needed for a majority Count 1 st -choice votes for each candidate: A = 10 < 16 = (NOT a majority) B = = 9 < 16 = (NOT a majority) D = = 12 < 16 = (NOT a majority) Josh Engwer (TTU) Voting Methods 15 July / 49
30 Plurality-with-Elimination Method (Example) WEX : Using Plurality-with-Elimination Mtd, determine election winner: 1 st choice A D B D B 2 nd choice B B A A D 3 rd choice D A D B A Total votes = 31 At least 16 votes needed for a majority Count 1 st -choice votes for each candidate: A = 10 < 16 = (NOT a majority) B = = 9 < 16 = (NOT a majority) D = = 12 < 16 = (NOT a majority) Since no candidate has a majority, eliminate candidate(s) with fewest votes: B Josh Engwer (TTU) Voting Methods 15 July / 49
31 Plurality-with-Elimination Method (Example) WEX : Using Plurality-with-Elimination Mtd, determine election winner: 1 st choice A D B D B 2 nd choice B B A A D 3 rd choice D A D B A Total votes = 31 At least 16 votes needed for a majority Since no candidate has a majority, eliminate candidate(s) with fewest votes: B Josh Engwer (TTU) Voting Methods 15 July / 49
32 Plurality-with-Elimination Method (Example) WEX : Using Plurality-with-Elimination Mtd, determine election winner: 1 st choice A D B D B 2 nd choice B B A A D 3 rd choice D A D B A Total votes = 31 At least 16 votes needed for a majority Eliminate candidate B, moving every entry below B (in blue) up one row Josh Engwer (TTU) Voting Methods 15 July / 49
33 Plurality-with-Elimination Method (Example) WEX : Using Plurality-with-Elimination Mtd, determine election winner: 1 st choice A D A D D 2 nd choice D A D A A 3 rd choice Total votes = 31 At least 16 votes needed for a majority Eliminate candidate B, moving every entry below B (in blue) up one row Josh Engwer (TTU) Voting Methods 15 July / 49
34 Plurality-with-Elimination Method (Example) WEX : Using Plurality-with-Elimination Mtd, determine election winner: 1 st choice A D A D D 2 nd choice D A D A A Total votes = 31 At least 16 votes needed for a majority Round 3 Josh Engwer (TTU) Voting Methods 15 July / 49
35 Plurality-with-Elimination Method (Example) WEX : Using Plurality-with-Elimination Mtd, determine election winner: 1 st choice A D A D D 2 nd choice D A D A A Total votes = 31 At least 16 votes needed for a majority Count 1 st -choice votes for each candidate: A = = 15 < 16 = (NOT a majority) D = = = (MAJORITY!) Josh Engwer (TTU) Voting Methods 15 July / 49
36 Plurality-with-Elimination Method (Example) WEX : Using Plurality-with-Elimination Mtd, determine election winner: 1 st choice A D A D D 2 nd choice D A D A A Total votes = 31 At least 16 votes needed for a majority Count 1 st -choice votes for each candidate: A = = 15 < 16 = (NOT a majority) D = = = (MAJORITY!) Since candidate D has a majority, D is the winner Josh Engwer (TTU) Voting Methods 15 July / 49
37 Pairwise Comparison Method (Definition) The election winner is expected to beat each candidate head-to-head : Definition (Pairwise Comparison Method) SETUP: Single-Winner Election has k candidates PROCESS: (1) Voters rank all candidates (2) Pit candidates A and B head-to-head Count how many voters prefer A to B Count how many voters prefer B to A If A and B are tied, then each receives 1/2 point Else the more preferred candidate receives 1 point and the less preferred candidate receives 0 points (3) Repeat Step (2) for each pair of candidates WINNER: Candidate receiving the most points Pairwise Comparison Method is typically used in Sports Drafts. Josh Engwer (TTU) Voting Methods 15 July / 49
38 How Many Pairwise Comparisons are Necessary? # CANDIDATES TOTAL # PAIRWISE COMPARISONS Josh Engwer (TTU) Voting Methods 15 July / 49
39 Pairwise Comparisons Method (Example) WEX : Using Pairwise Comparisons Mtd, determine election winner: 1 st choice A D B C B 2 nd choice C B C D C 3 rd choice B A A A D 4 th choice D C D B A Josh Engwer (TTU) Voting Methods 15 July / 49
40 Pairwise Comparisons Method (Example) WEX : Using Pairwise Comparisons Mtd, determine election winner: 1 st choice A D B C B 2 nd choice C B C D C 3 rd choice B A A A D 4 th choice D C D B A CANDIDATE POINTS 1 2 -POINTS A B 1 C D A versus B: A = = 15 B = = 16 WINNER! Josh Engwer (TTU) Voting Methods 15 July / 49
41 Pairwise Comparisons Method (Example) WEX : Using Pairwise Comparisons Mtd, determine election winner: 1 st choice A D B C B 2 nd choice C B C D C 3 rd choice B A A A D 4 th choice D C D B A CANDIDATE POINTS 1 2 -POINTS A 1 B 1 C D A versus C: A = = 17 WINNER! C = = 14 Josh Engwer (TTU) Voting Methods 15 July / 49
42 Pairwise Comparisons Method (Example) WEX : Using Pairwise Comparisons Mtd, determine election winner: 1 st choice A D B C B 2 nd choice C B C D C 3 rd choice B A A A D 4 th choice D C D B A CANDIDATE POINTS 1 2 -POINTS A 1 B 1 C D 1 A versus D: A = = 15 D = = 16 WINNER! Josh Engwer (TTU) Voting Methods 15 July / 49
43 Pairwise Comparisons Method (Example) WEX : Using Pairwise Comparisons Mtd, determine election winner: 1 st choice A D B C B 2 nd choice C B C D C 3 rd choice B A A A D 4 th choice D C D B A CANDIDATE POINTS 1 2 -POINTS A 1 B 2 C D 1 B versus C: B = = 16 WINNER! C = = 15 Josh Engwer (TTU) Voting Methods 15 July / 49
44 Pairwise Comparisons Method (Example) WEX : Using Pairwise Comparisons Mtd, determine election winner: 1 st choice A D B C B 2 nd choice C B C D C 3 rd choice B A A A D 4 th choice D C D B A CANDIDATE POINTS 1 2 -POINTS A 1 B 3 C D 1 B versus D: B = = 19 WINNER! D = = 12 Josh Engwer (TTU) Voting Methods 15 July / 49
45 Pairwise Comparisons Method (Example) WEX : Using Pairwise Comparisons Mtd, determine election winner: 1 st choice A D B C B 2 nd choice C B C D C 3 rd choice B A A A D 4 th choice D C D B A CANDIDATE POINTS 1 2 -POINTS A 1 B 3 C 1 D 1 C versus D: C = = 24 WINNER! D = 7 Josh Engwer (TTU) Voting Methods 15 July / 49
46 Pairwise Comparisons Method (Example) WEX : Using Pairwise Comparisons Mtd, determine election winner: 1 st choice A D B C B 2 nd choice C B C D C 3 rd choice B A A A D 4 th choice D C D B A CANDIDATE POINTS 1 2 -POINTS A 1 B 3 C 1 D 1 Since candidate B has the most points, B is the winner Josh Engwer (TTU) Voting Methods 15 July / 49
47 Summary of the Previous Examples 1 st choice A D B C B 2 nd choice C B C D C 3 rd choice B A A A D 4 th choice D C D B A EXAMPLE VOTING METHOD WINNER WEX Plurality A WEX Borda Count C WEX Plurality+Elimination D WEX Pairwise Comparisons B As this shows, it s possible for each method to determine a different winner of the same election. Josh Engwer (TTU) Voting Methods 15 July / 49
48 Voter Apathy & Alternative Voting Methods Definition (Voter Apathy) Voter apathy is the belief that one s vote does not count. Some voter apathy is caused by frustration with the Plurality Method. In response to voter apathy, alternative voting methods have been proposed: Approval Voting Instant Runoff Voting Voters vote for as many candidates as they want Voters rank candidates Weakest candidate is eliminated If eliminated candidate was a voter s 1 st choice, then that candidate becomes voter s 2 nd choice (i.e vote was not wasted) Such alternative voting methods are beyond the scope of this course. Josh Engwer (TTU) Voting Methods 15 July / 49
49 Fin Fin. Josh Engwer (TTU) Voting Methods 15 July / 49
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