Election Score Can Be Harder Than Winner

TL;DR

This paper reveals that in certain election systems, determining the winner can be computationally easier than calculating the candidate's score, especially under specific preference restrictions like dichotomous and single-peaked preferences.

Contribution

It uncovers unusual complexity behaviors in election systems, showing that winner determination can be easier than score computation, and provides a polynomial-time algorithm for Dodgson score in single-peaked preferences.

Findings

Winner problem is easy for dichotomous Young elections.

Score computation is hard for the same elections.

Provides polynomial-time algorithm for Dodgson score in single-peaked electorates.

Abstract

Election systems based on scores generally determine the winner by computing the score of each candidate and the winner is the candidate with the best score. It would be natural to expect that computing the winner of an election is at least as hard as computing the score of a candidate. We show that this is not always the case. In particular, we show that for Young elections for dichotomous preferences the winner problem is easy, while determining the score of a candidate is hard. This complexity behavior has not been seen before and is unusual. The easiness of the winner problem for dichotomous Young crucially uses the fact that dichotomous preferences guarantee the transitivity of the majority relation. In addition to dichotomous preferences we also look at single-peaked preferences, the most well-studied domain restriction that guarantees the transitivity of the majority relation. We show that for the three major hard election systems and their natural variants, dichotomous Young is the only case where winner is easy and score is hard. This also solves an open question from Lackner and Peters (AAAI 2017), by providing a polynomial-time algorithm for Dodgson score for single-peaked electorates.