1. Introduction
In recent years, an increase in time-basedcompetition andthe growth of e-commerce have put
pressure on warehouse managers to streamline warehouse operations (Shamlaty [1]). Trade journals
are full of success stories illustrating their solutions to warehouse design and control problems
(e.g. Maloney [2], Ruriani [3]). These problems include warehouse layout, stock pro2ling, picking
strategies andhow to set stock location. The focus of this paper is on order picking. Coyle et al. [4]
point out that up to 65% of the operating costs of a warehouse can be attributedto order picking.