voting paradoxes and how to deal with them hannu nurmi university of turku turku, finland
DESCRIPTION
PARADOXES OCCUR 1992 ELECTION –Bush and Poirot win popular election 2000 Election –Bush II loses popular vote, wins election They happen every day in the rack/stack method used in DoDTRANSCRIPT
VOTING PARADOXES
AND HOW TO
DEAL WITH THEM
Hannu NurmiUniversity of
TurkuTurku, Finland
VOTING
• Satisfaction and justice in voting outcomes is important
• Every day, somebody is rackin’ and stackin’
• Voting is a way to reach equitable consensus
PARADOXES OCCUR
• 1992 ELECTION– Bush and Poirot win popular election
• 2000 Election– Bush II loses popular vote, wins
election• They happen every day in the
rack/stack method used in DoD
ASSUMPTIONS
• Equal Weight• One Vote Each• Independence (no gaming)• Transitivity (A < B and B < C implies A < C)
• DEFN: An Alternative is one of the choices• NOTATION: a > b means a is prefered to b
PREFERENCE PROFILE
COUNT
3 4 2 7 5 6
1ST A B C A B C
2ND B C A C A B
3RD C A B B C A
WHO WINS?1ST PLACE VOTES
A 3+7 10B 4+5 9C 2+6 8
LAST PLACE VOTESA 4+6 10B 2+7 9C 3+5 8
TOP TWOA 3+2+7+5 19B 3+4+5+6 18C 4+2+7+7 16
A B CA 12 15B 15 12C 12 15
A B CA 0 1B 1 0C 0 1
TOURNAMENT MATRIX
PAIRWISE COMPARISON MATRIXfor 12 voters, B>A (note: nontransitivity)
CONDORSET WINNERS AND LOSERS
• A < B, 13 vs. 8• A < C, 13 vs 8• B < C, 13 vs. 8• But, A wins
plurality vote!• A is the Condorcet
loser– uniformly despised
1 7 7 6A A B CB C C BC B A A
BORDA (1770)
• give k points to last place• give k + a points for second to last• give k + 2a points for third from last• etc.
• Borda never elects the Condorcet loser• Does Not always elect the Condorcet
winner
SUMMED RANKIs the usual bad?
• One (1) point for first place• Two (2) points for second place• etc.
• Sum the point scores• Select the alternative with the
lowest score
ANALYSIS
• Reverse the ranks• k = 1• a = 1
• Always selects the Condorcet winner if it exists
• May select Condorcet loser if it exists
VOTING PARADOXES
• What follows is a set of situations where the vote fails to reflect consensus. Many of these situations are famous.
NO SHOW PARADOX26% 47% 2% 25%
A B B CB C C AC A A B
• Plurality run-off voting• 1st Round: Eliminate C
– A wins in run-off with 51%• Suppose the 47% no-show
– B is eliminated, C subsequently beats A– the 47% get their second choice, not their 3rd
INCONSISTENCY PARADOX
east east east west west west35% 40% 25% 40% 55% 5%
A B C C B AB C B B C CC A A A A B
• Plurality run-off voting in each district• B wins the East in run-off, wins West
outright• Taken as a whole, C beats B in a run-off
ALABAMA PARADOX OF 1881Hamiltonian Apportionment
TOTAL SEATS 299 300
ALABAMA 7.646 7.671TEXAS 9.64 9.672
ILLINOIS 18.64 18.7
ALABAMA SEATS 8 7
• Seats allocated by integer part, remainder allocated by largest fraction remaining
seatsseat
poppop ii
OSTRAGORSKI’s PARADOX
Arises because the following two produce different winners:
1. BEAUTY CONTEST: Each voter votes for the candidate whose stand is closest to his in a majority of issues.
2. ISSUE CONTEST: For each issue, voters pick candidates. The winner is the one winning the majority of issues.
BEAUTY WINNER
A X X X X
B X Y X X
C Y X X X
D Y Y Y Y
E Y Y Y Y
ISSUE WINNER Y Y X
SIMPSON’s REPRESENTATION PARADOX
• Percent who favor higher in the East for both employed and unemployed
• Total percent in favor larger in the West
EAST WEST EAST WEST EAST WEST
EMPLOYED 400,000 90,000 80,000 15,000 20% 17%
UNEMPLOYED 100,000 80,000 50,000 35,000 50% 44%
total 500,000 170,000 130,000 50,000 26% 29%
POPULATIONFAVOR
INITIATIVE