Ties Matter: Complexity of Manipulation when Tie-Breaking with a Random Vote

H. Aziz, S. Gaspers, N. Mattei, N. Narodytska, T. Walsh
AAAI 2013
Abstract
We study the impact on strategic voting of tie-breaking by means of considering the order of tied candidates within a random vote. We compare this to another non deterministic tie-breaking rule where we simply choose candidate uniformly at random. In general, we demonstrate that there is no connection between the computational complexity of computing a manipulating vote with the two different types of tie-breaking. However, we prove that for some scoring rules, the computational complexity of computing a manipulation can increase from polynomial to NP-hard. We also discuss the relationship with the computational complexity of computing a manipulating vote when we ask for a candidate to be the unique winner, or to be among the set of co-winners.

Experiments:

Election type Culture Candidates Voters Instances Parameters
Ordinal Impartial Anonymous Culture {3, 4, 8, 16, 32, 64, 128} {1, 2, 3, 4, 5, 8, 9, 16, 17, 32, 33, 64, 65, 128, 129} 1000 None
Ordinal Impartial Culture {3, 4, 8, 16, 32, 64, 128} {1, 2, 3, 4, 5, 8, 9, 16, 17, 32, 33, 64, 65, 128, 129} 1000 None
Ordinal Single-Peaked (Walsh/Uniform) {3, 4, 8, 16, 32, 64, 128} {1, 2, 3, 4, 5, 8, 9, 16, 17, 32, 33, 64, 65, 128, 129} 1000 None
Ordinal Urn Model {3, 4, 8, 16, 32, 64, 128} {1, 2, 3, 4, 5, 8, 9, 16, 17, 32, 33, 64, 65, 128, 129} 1000 replacement was set to $m! - 1$ ($b=m!-1$?)