Analyzing the Practical Relevance of Voting Paradoxes via Ehrhart Theory, Computer Simulations, and Empirical Data

F. Brandt, C. Geist, M. Strobel
AAMAS 2016
Abstract
Results from social choice theory are increasingly used to argue about collective decision making in computational multiagent systems. A large part of the social choice literature studies voting paradoxes in which seemingly mild properties are violated by common voting rules. In this paper, we investigate the likelihood of the Condorcet Loser Paradox (CLP) and the Agenda Contraction Paradox (ACP) using Ehrhart theory, computer simulations, and empirical data. We present the first analytical results for the CLP on four alternatives and show that our experimental results, which go well beyond four alternatives, are in almost perfect congruence with the analytical results. It turns out that the CLP---which is often cited as a major flaw of some Condorcet extensions such as Dodgson's rule, Young's rule, and MaxiMin---is of no practical relevance. The ACP, on the other hand, frequently occurs under various distributional assumptions about the voters' preferences. The extent to which it is real threat, however, strongly depends on the voting rule, the underlying distribution of preferences, and, somewhat surprisingly, the parity of the number of voters.

Remarks: Has a journal version with different title and behind a paywall: Analyzing the Practical Relevance of the Condorcet Loser Paradox and the Agenda Contraction Paradox (https://link.springer.com/chapter/10.1007/978-3-030-48598-6_5)

Experiments:

Election type Culture Candidates Voters Instances Parameters
Ordinal Euclidean 2D {4} [1-30] 100000 Uniform 2D
Ordinal Euclidean 2D {4} [49-51] 100000 Uniform 2D
Ordinal Euclidean 2D {4} [99-101] 100000 Uniform 2D
Ordinal Euclidean 2D {4} [199-201] 100000 Uniform 2D
Ordinal Euclidean 2D {4} [499-501] 100000 Uniform 2D
Ordinal Euclidean 2D {4} [999-1001] 100000 Uniform 2D
Ordinal Impartial Anonymous Culture {4} [1-30] 100000 None
Ordinal Impartial Anonymous Culture {4} [49-51] 100000 None
Ordinal Impartial Anonymous Culture {4} [99-101] 100000 None
Ordinal Impartial Anonymous Culture {4} [199-201] 100000 None
Ordinal Impartial Anonymous Culture {4} [499-501] 100000 None
Ordinal Impartial Anonymous Culture {4} [999-1001] 100000 None
Ordinal Impartial Culture {4} [1-30] 100000 None
Ordinal Impartial Culture {4} [49-51] 100000 None
Ordinal Impartial Culture {4} [99-101] 100000 None
Ordinal Impartial Culture {4} [199-201] 100000 None
Ordinal Impartial Culture {4} [499-501] 100000 None
Ordinal Impartial Culture {4} [999-1001] 100000 None
Ordinal Mallows {4} [1-30] 100000 0.8
Ordinal Mallows {4} [49-51] 100000 0.8
Ordinal Mallows {4} [99-101] 100000 0.8
Ordinal Mallows {4} [199-201] 100000 0.8
Ordinal Mallows {4} [499-501] 100000 0.8
Ordinal Mallows {4} [999-1001] 100000 0.8
Ordinal PrefLib None None 100000 *
Ordinal Urn Model {4} [1-30] 100000 $\alpha = 10/m!$
Ordinal Urn Model {4} [49-51] 100000 $\alpha = 10/m!$
Ordinal Urn Model {4} [99-101] 100000 $\alpha = 10/m!$
Ordinal Urn Model {4} [199-201] 100000 $\alpha = 10/m!$
Ordinal Urn Model {4} [499-501] 100000 $\alpha = 10/m!$
Ordinal Urn Model {4} [999-1001] 100000 $\alpha = 10/m!$
Ordinal Euclidean 2D [2-10] {50, 51} 1000000 Uniform 2D
Ordinal Impartial Anonymous Culture [2-10] {50, 51} 1000000 None
Ordinal Impartial Culture [2-10] {50, 51} 1000000 None
Ordinal Mallows [2-10] {50, 51} 1000000 0.8
Ordinal Urn Model [2-10] {50, 51} 1000000 10