On Coalitional Manipulation for Multiwinner Elections: Shortlisting

TL;DR

This paper analyzes the computational complexity of strategic voting in multiwinner shortlisting elections using the l-Bloc rule, considering various tie-breaking and evaluation methods, revealing intractability in some cases.

Contribution

It provides the first detailed complexity analysis of strategic voting in shortlisting under l-Bloc, exploring effects of tie-breaking and evaluation functions.

Findings

Strategic voting can be computationally intractable in egalitarian settings.

Tie-breaking mechanisms influence the complexity landscape.

The study offers a comprehensive view of complexity in this neglected area.

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, l-Bloc (every voter approves l candidates). In particular, we investigate the influence of several tie-breaking mechanisms (e.g., pessimistic versus optimistic) and group evaluation functions (e.g., egalitarian versus utilitarian). Among other things, 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 so far in the literature of this neglected scenario.