On Hill's Worst-Case Guarantee for Indivisible Bads

TL;DR

This paper extends Hill's worst-case guarantee approach to the allocation of indivisible bads, providing a tight closed-form characterization of the MinMaxShare and demonstrating its effectiveness and optimality within Hill's model.

Contribution

It applies Hill's guarantee concept to bads, derives a closed-form solution for the worst-case MinMaxShare, and proves the monotonic cover as the best possible guarantee in this setting.

Findings

Derived a closed-form expression for the worst-case MinMaxShare for bads.

Showed Hill's guarantee is close to the original MinMaxShare in effectiveness.

Proved the monotonic cover of Hill's share is the optimal guarantee in Hill's model.

Abstract

When allocating objects among agents with equal rights, people often evaluate the fairness of an allocation rule by comparing their received utilities to a benchmark share - a function only of her own valuation and the number of agents. This share is called a guarantee if for any profile of valuations there is an allocation ensuring the share of every agent. When the objects are indivisible goods, Budish [J. Political Econ., 2011] proposed MaxMinShare, i.e., the least utility of a bundle in the best partition of the objects, which is unfortunately not a guarantee. Instead, an earlier pioneering work by Hill [Ann. Probab., 1987] proposed for a share the worst-case MaxMinShare over all valuations with the same largest possible single-object value. Although Hill's share is more conservative than the MaxMinShare, it is an actual guarantee and its computation is elementary, unlike that of the MaxMinShare which involves solving an NP-hard problem. We apply Hill's approach to the allocation of indivisible bads (objects with disutilities or costs), and characterise the tight closed form of the worst-case MinMaxShare for a given value of the worst bad. We argue that Hill's share for allocating bads is effective in the sense of being close to the original MinMaxShare value, and there is much to learn about the guarantee an agent can be offered from the disutility of her worst single bad. Furthermore, we prove that the monotonic cover of Hill's share is the best guarantee that can be achieved in Hill's model for all allocation instances.