Fixed‐interval joint‐replenishment policies for distribution systems with multiple retailers and stochastic demand

We consider a distribution system consisting of a central warehouse and a group of retailers facing independent stochastic demand. The retailers replenish from the warehouse, and the warehouse from an outside supplier with ample supply. Time is continuous. Most previous studies on inventory control policies for this system have considered stock‐based batch‐ordering policies. We develop a time‐based joint‐replenishment policy in this study. Let the warehouse set up a basic replenishment interval. The retailers are replenished through the warehouse in intervals that are integer multiples of the basic replenishment interval. No inventory is carried at the warehouse. We provide an exact evaluation of the long‐term average system costs under the assumption that stock can be balanced among the retailers. The structural properties of the inventory system are characterized. We show that, although it is well known that stock‐based inventory control policies dominate time‐based inventory control policies at a single facility, this dominance does not hold for distribution systems with multiple retailers and stochastic demand. This is because the latter can provide a more efficient mechanism to streamline inventory flow and pool retailer demand, even though the former may be able to use more updated stock information to optimize system performance. The findings of the study provide insights about the key factors that drive the performance of a multiechelon inventory control system. © 2013 Wiley Periodicals, Inc. Naval Research Logistics 60: 637–651, 2013     

How Different Terrorist Attacks Affect Stock Markets

Terrorist attacks adversely affect the Pakistani stock market. However, such effect is short-lived: the market recovers from terrorist shocks in one day. The impact of attack depends on the locations and types of attack. The more severe the attack (i.e. more people killed), the more negative is the KSE-100 index return. Most interestingly, stock market contains information about future attacks. In sum, different tactics of terrorists have varied effects on financial markets, which in turn can predict terrorist attacks.     

Performance evaluation of a production/inventory system with periodic review and endogenous lead times

This paper considers a discrete time, single item production/inventory system with random period demands. Inventory levels are reviewed periodically and managed using a base‐stock policy. Replenishment orders are placed with the production system which is capacitated in the sense that there is a single server that sequentially processes the items one at a time with stochastic unit processing times. In this setting the variability in demand determines the arrival pattern of production orders at the queue, influencing supply lead times. In addition, the inventory behavior is impacted by the correlation between demand and lead times: a large demand size corresponds to a long lead time, depleting the inventory longer. The contribution of this paper is threefold. First, we present an exact procedure based on matrix‐analytic techniques for computing the replenishment lead time distribution given an arbitrary discrete demand distribution. Second, we numerically characterize the distribution of inventory levels, and various other performance measures such as fill rate, base‐stock levels and optimal safety stocks, taking the correlation between demand and lead times into account. Third, we develop an algorithm to fit the first two moments of the demand and service time distribution to a discrete phase‐type distribution with a minimal number of phases. This provides a practical tool to analyze the effect of demand variability, as measured by its coefficient of variation, on system performance. We also show that our model is more appropriate than some existing models of capacitated systems in discrete time. © 2007 Wiley Periodicals, Inc. Naval Research Logistics, 2007     

震灾应急物资动态需求预测模型研究

为提高应急物资需求预测方法的实用性,同时考虑提前期和需求的不确定性,采用安全库存理论构建震灾应急物资动态需求预测模型,以观测得到的实际到货量、实际消耗量和实时剩余库存量等信息来动态预测当前时刻应急物资的需求量。应用算例表明,所建模型不仅能够准确反映需求量与被保障人员总数的正相关关系,而且还能实时体现被保障人员总数的变化情况。因此,该模型能够为震灾过程中应急物资需求量的确定提供科学决策依据,具有一定的实用价值和应用前景。     

装甲装备器材库存超(欠)储状态分析与程度评价

定义了装甲装备器材库存超(欠)储的概念,提出了维修保障资源点器材储存状态的定性判别和量化方法;考虑数量、时间、空间等影响因素,提出了资源点维修器材超(欠)储程度的评价指标和评价方法,并结合某军区实际数据进行了验证。结果表明:资源点的平均超(欠)储程度反映了保障体系内所有资源点库存偏离正常水平的平均程度,可为确定调剂资源点的优先次序和供应数量提供依据。     

基于信息态势共享的编队反导目标分配方案优化

在编队目标信息态势实现共享的条件下,可实现网络化防空反导作战的常规作战样式,对目标进行集中统一分配,如何形成最优化分配方案,是防空反导作战指挥决策中的关键。在给定目标来袭态势下,根据编队目标分配原则,可形成多种目标分配初始方案,先后采用了基于负载平衡和基于任务有利度方法,对目标分配方案进行优化。优化所得方案有利度的均方根最大,目标来袭密度的均方根最小,可作为下一步编队组织使用火力通道的依据。     

Production‐inventory systems with imperfect advance demand information and updating

We consider a supplier with finite production capacity and stochastic production times. Customers provide advance demand information (ADI) to the supplier by announcing orders ahead of their due dates. However, this information is not perfect, and customers may request an order be fulfilled prior to or later than the expected due date. Customers update the status of their orders, but the time between consecutive updates is random. We formulate the production‐control problem as a continuous‐time Markov decision process and prove there is an optimal state‐dependent base‐stock policy, where the base‐stock levels depend upon the numbers of orders at various stages of update. In addition, we derive results on the sensitivity of the state‐dependent base‐stock levels to the number of orders in each stage of update. In a numerical study, we examine the benefit of ADI, and find that it is most valuable to the supplier when the time between updates is moderate. We also consider the impact of holding and backorder costs, numbers of updates, and the fraction of customers that provide ADI. In addition, we find that while ADI is always beneficial to the supplier, this may not be the case for the customers who provide the ADI. © 2011 Wiley Periodicals, Inc. Naval Research Logistics, 2011     

Optimal control of a capacitated inventory system with multiple demand classes

This article studies the optimal control of a periodic‐review make‐to‐stock system with limited production capacity and multiple demand classes. In this system, a single product is produced to fulfill several classes of demands. The manager has to make the production and inventory allocation decisions. His objective is to minimize the expected total discounted cost. The production decision is made at the beginning of each period and determines the amount of products to be produced. The inventory allocation decision is made after receiving the random demands and determines the amount of demands to be satisfied. A modified base stock policy is shown to be optimal for production, and a multi‐level rationing policy is shown to be optimal for inventory allocation. Then a heuristic algorithm is proposed to approximate the optimal policy. The numerical studies show that the heuristic algorithm is very effective. © 2011 Wiley Periodicals, Inc. Naval Research Logistics 58: 43–58, 2011     

Base‐stock policies in capacitated assembly systems: Convexity properties

We study an assembly system with a single finished product managed using an echelon base‐stock or order‐up‐to policy. Some or all operations have capacity constraints. Excess demand is either backordered in every period or lost in every period. We show that the shortage penalty cost over any horizon is jointly convex with respect to the base‐stock levels and capacity levels. When the holding costs are also included in the objective function, we show that the cost function can be written as a sum of a convex function and a concave function. Throughout the article, we discuss algorithmic implications of our results for making optimal inventory and capacity decisions in such systems.© 2009 Wiley Periodicals, Inc. Naval Research Logistics, 2010     

装备器材多级库存决策优化模型研究

研究了装备器材管理的成本构成特点,认为装备器材多极库存系统可简化为n级系列体系结构。建立了装备器材多级库存优化控制模型,通过模型求解和相应实例分析,表明了装备器材多级库存优化控制方法的可行性和有效性。