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W e study an assembly system with a number of parallel multistage processes feeding a multistage final assembly process. Each stage has a stochastic throughput time. We assume that the system is controlled by planned leadtimes at each stage. From these planned leadtimes the start and due times of all stages can be derived. If a job finishes at a particular stage and has to wait before the start of the next job(s), a holding cost proportional to the waiting time is incurred. A penalty cost proportional to the lateness is incurred when the last stage of the final assembly process finishes after its due time. The objective is to determine planned leadtimes for each individual stage, such that the expected cost of a customer order is minimized.We derive the recursive equations for the tardiness and earliness at all stages and an exact expression for the expected cost. We discuss the similarity between these expressions and those for serial inventory systems. Based on this observation and a conjecture related to the generalized Newsvendor equations, we develop an iterative heuristic procedure. Comparison with a numerical optimization method confirms the accuracy of the heuristic. Finally, we discuss an application of the model to a real-life case, showing the added value of a system-wide optimization of planned leadtimes compared to current practice.

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We consider a firm which faces a Poisson customer demand and uses a base-stock policy to replenish its inventories from an outside supplier with a fixed lead time. The firm can use a preorder strategy which allows the customers to place their orders before their actual need. The time from a customer's order until the date a product is actually needed is called commitment lead time. The firm pays a commitment cost which is strictly increasing and convex in the length of the commitment lead time. For such a system, we prove the optimality of bang-bang and all-or-nothing policies for the commitment lead time and the base-stock policy, respectively. We study the case where the commitment cost is linear in the length of the commitment lead time in detail. We show that there exists a unit commitment cost threshold which dictates the optimality of either a buy-to-order (BTO) or a buy-to-stock strategy. The unit commitment cost threshold is increasing in the unit holding and backordering costs and decreasing in the mean lead time demand. We determine the conditions on the unit commitment cost for profitability of the BTO strategy and study the case with a compound Poisson customer demand. KEYWORDS advance demand information, commitment cost, inventory management, preorder strategy 1 This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.