Abstract
Shortlisting of candidates—selecting a group of “best” candidates—is a special case of multiwinner elections. We provide the first in-depth study of the computational complexity of strategic voting for shortlisting based on the perhaps most basic voting rule in this scenario, ell -Bloc (every voter approves ell candidates). In particular, we investigate the influence of several different group evaluation functions (e.g., egalitarian versus utilitarian) and tie-breaking mechanisms modeling pessimistic and optimistic manipulators. Among other things, we conclude that in an egalitarian setting strategic voting may indeed be computationally intractable regardless of the tie-breaking rule. Altogether, we provide a fairly comprehensive picture of the computational complexity landscape of this scenario.
Highlights
Assume that a university wants to select the two favorite pieces in classical style to be played during the graduation ceremony
We focus on the family of -Bloc multiwinner voting rules, that is a family of scoring rules that assign, for each vote, one point to each of the top < |C| candidates
We show that -Bloc-F -eval-Coalitional Manipulation can be solved in polynomial time for any ∈ N, any eval ∈ { util, candegal }, and any tie-breaking rule
Summary
Assume that a university wants to select the two favorite pieces in classical style to be played during the graduation ceremony. The students were asked to submit their favorite pieces. A jury consisting of seven members (three juniors and four seniors). A preliminary version of this article appeared in the Proceedings of the Twenty-Sixth International Joint Conference on Artificial Intelligence (IJCAI ’17) [12]. In this full version we included all proofs and algorithms (together with our ILP formulations). We formalized the concept of simulation among tie-breaking rules
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