Papers › Metric-Distortion Bounds under Limited Information

Metric-Distortion Bounds under Limited Information

6 Jul 2021arXiv:2107.02489links table onlyarchive 2025-07-28

Ioannis Anagnostides, Dimitris Fotakis, Panagiotis Patsilinakos

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In this work we study the metric distortion problem in voting theory under a limited amount of ordinal information. Our primary contribution is threefold. First, we consider mechanisms which perform a sequence of pairwise comparisons between candidates. We show that a widely-popular deterministic mechanism employed in most knockout phases yields distortion 𝒪(logm) while eliciting only m-1 out of Θ(m²) possible pairwise comparisons, where m represents the number of candidates. Our analysis for this mechanism leverages a powerful technical lemma recently developed by Kempe \cite{DBLP:conf/aaai/000120a}. We also provide a matching lower bound on its distortion. In contrast, we prove that any mechanism which performs fewer than m-1 pairwise comparisons is destined to have unbounded distortion. Moreover, we study the power of deterministic mechanisms under incomplete rankings. Most notably, when every agent provides her k-top preferences we show an upper bound of 6 m/k + 1 on the distortion, for any k ∈{1, 2, …, m}. Thus, we substantially improve over the previous bound of 12 m/k recently established by Kempe \cite{DBLP:conf/aaai/000120a,DBLP:conf/aaai/000120b}, and we come closer to matching the best-known lower bound. Finally, we are concerned with the sample complexity required to ensure near-optimal distortion with high probability. Our main contribution is to show that a random sample of Θ(m/ϵ²) voters suffices to guarantee distortion 3 + ϵ with high probability, for any sufficiently small ϵ> 0. This result is based on analyzing the sensitivity of the deterministic mechanism introduced by Gkatzelis, Halpern, and Shah \cite{DBLP:conf/focs/Gkatzelis0020}. Importantly, all of our sample-complexity bounds are distribution-independent.

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eurovision_instance_opt_voting ioannisAnagno/Voting-MetricDistortion/eurovision_voting.py official repository unverified MIT (permissive) · f03d0481177730d0 · report
instance_opt_decisive_voting ioannisAnagno/Voting-MetricDistortion/instance_opt_decisive.py official repository unverified MIT (permissive) · 998733bb059c32e1 · report
instance_opt_voting ioannisAnagno/Voting-MetricDistortion/instance_opt.py official repository unverified MIT (permissive) · f5796e4f4c68b458 · report

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