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How to Rig Elections and Competitions Noam Hazon*, Paul E. Dunne +, Sarit Kraus*, Michael Wooldridge + + Dept. of Computer Science University of Liverpool United Kingdom *Dept. of Computer Science Bar Ilan University Israel

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Outline A common way to aggregate agents preferences - voting But it can be manipulated! Current models assume perfect information We analyzed a model of imperfect information Theoretically, manipulation is hard: NP-Complete proofs Practically it is not: Heuristics and experimental evaluation Conclusion, Future work

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Why Voting? Preference aggregation - to a socially desirable decision Not only agents… Human decisions among a collection of alternatives Sport tournaments But it can be manipulated if we have perfect information By a single voter or a coalition of voters By the election officer

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Our Model - Imperfect Information Agenda rigging by the election officer Linear order Tree order Complexity results Verifying an agenda is in P Finding a perfect agenda seems to be hard Empirical results Easy-to-implement heuristics performed well Tested against random and real data Motivation - better design of voting protocols

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Set of outcomes/candidates, Imperfect information ballot matrix M, Voting trees Basic Definitions ABCD CA C CD B AB C A

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Linear Order Tree Verifying an agenda is O(n 2 ) - with dynamic programming A candidate can only benefit by going late in an order And this is not true for general trees! Finding an order for a particular candidate to win: With a non-zero probability - O(n 2 ) With at least a certain probability

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Weak Version of Rigged Agenda A run - An order agenda together with the intended outcomes P[r | M] - a probability that the run will result in our candidate WIIRA Problem - Set of candidates, Ω Imperfect information matrix, M A favored candidate, ω* A probability p Is there a run in which the winner is ω*, s.t. P[r | M] p ? NP-C, from the k-HCA problem on tournaments

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Balanced Tree Order Verifying an agenda is O(n 2 ) Finding an order for a particular candidate to win: With a non-zero probability - open problem [Lang07] With at least a certain probability - NP-C [Vu08]

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From Theory to Practice Concern: It might be that there are effective heuristics to manipulate an election, even though manipulation is NP-complete. Discussion: True. The existence of effective heuristics would weaken any practical import of our idea. It would be very interesting to find such heuristics. [Bartholdi89]

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Linear Order – Conversion Probabilistic data deterministic data Simple convert Threshold convert Not better than random order ω3ω3ω2ω2ω1ω1 0.60.7- ω1ω1 -0.3 ω2ω2 - 0.4 ω3ω3 ω3ω3ω2ω2ω1ω1 01- ω1ω1 1-0 ω2ω2 -00 ω3ω3

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General Approach Heuristics: Far adversary.... Best win Local search Random order Two data sets: Random data- uniform and normal distribution Real data - 29 teams from the NBA, top 13 tennis players ω*ω*m-1m-212

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Linear Order- Normal Distribution

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Linear Order- 13 Tennis Players

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Balanced Tree- Normal Distribution

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Balanced Tree- 13 Tennis Players

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Balanced Tree- 29 NBA Teams

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Conclusion Two voting protocols with incomplete information Agenda verification is in P Manipulation by the officer seems to be hard Far adversary, best win and local search can help a lot! Future: other voting protocols {hazonn,sarit}@cs.biu.ac.il, {mjw,ped}@csc.liv.ac.uk

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