On the Complexity of Winner Determination and Strategic Control in Conditional Approval Voting

TL;DR

This paper analyzes the computational complexity of a generalized approval voting rule called Conditional Minisum (CMS), identifying conditions for efficient algorithms, approximation strategies, and resistance to strategic control in multi-issue elections.

Contribution

It introduces polynomial algorithms for CMS under bounded treewidth, develops approximation algorithms for restricted ballots, and studies the rule's resistance to strategic control.

Findings

Bounded treewidth condition enables polynomial algorithms for CMS.

Natural restrictions allow the first multiplicative approximation algorithms.

CMS shows resistance to control in most strategic manipulation scenarios.

Abstract

We focus on a generalization of the classic Minisum approval voting rule, introduced by Barrot and Lang (2016), and referred to as Conditional Minisum (CMS), for multi-issue elections with preferential dependencies. Under this rule, voters are allowed to declare dependencies between different issues, but the price we have to pay for this higher level of expressiveness is that we end up with a computationally hard rule. Motivated by this, we first focus on finding special cases that admit efficient algorithms for CMS. Our main result in this direction is that we identify the condition of bounded treewidth (of an appropriate graph, emerging from the provided ballots) as the necessary and sufficient condition for exact polynomial algorithms, under common complexity assumptions. We then move to the design of approximation algorithms. For the (still hard) case of binary issues, we identify natural restrictions on the voters' ballots, under which we provide the first multiplicative approximation algorithms for the problem. The restrictions involve upper bounds on the number of dependencies an issue can have on the others and on the number of alternatives per issue that a voter can approve. Finally, we also investigate the complexity of problems related to the strategic control of conditional approval elections by adding or deleting either voters or alternatives and we show that in most variants of these problems, CMS is computationally resistant against control. Overall, we conclude that CMS can be viewed as a solution that achieves a satisfactory tradeoff between expressiveness and computational efficiency, when we have a limited number of dependencies among issues, while at the same time exhibiting sufficient resistance to control.