The most important performance measures in a warehouse are generally related to the time or effort required for order picking, i.e. the retrieval of items from the shelves and delivering them to the point where they will be picked up by the appropriate vehicle. Informally, these performance measures can be optimized by placing high-runner items near to the entrance of the warehouse, and by storing items that are often ordered together close to each other. Technically, this problem is a special case of correlated storage assignment, where the organizational structure behind the correlation among request probabilities of different items is known and can be exploited by a mathematical model. We present a mixed integer programming (MIP) formulation of the problem that can be solved by commercial software in practically relevant problem sizes.
We show that minimizing order cycle time (maximum of individual pickers’ times) and minimizing expected picking effort (sum of pickers’ times) are conflicting criteria. Our MIP allows controlling the trade-off between these criteria by minimizing a linear combination of the two, or minimizing one of them subject to an upper bound on the other. The novel strategy has been compared to classical cube-per-order index-based (COI) techniques in computational experiments, where it achieved an up to 36–38% improvement compared to COI, according either criterion.