To wait or not to wait: Optimal ordering under lead time uncertainty and forecast updating

There has been a dramatic increase over the past decade in the number of firms that source finished product from overseas. Although this has reduced procurement costs, it has increased supply risk; procurement lead times are longer and are often unreliable. In deciding when and how much to order, firms must consider the lead time risk and the demand risk, i.e., the accuracy of their demand forecast. To improve the accuracy of its demand forecast, a firm may update its forecast as the selling season approaches. In this article we consider both forecast updating and lead time uncertainty. We characterize the firm's optimal procurement policy, and we prove that, with multiplicative forecast revisions, the firm's optimal procurement time is independent of the demand forecast evolution but that the optimal procurement quantity is not. This leads to a number of important managerial insights into the firm's planning process. We show that the firm becomes less sensitive to lead time variability as the forecast updating process becomes more efficient. Interestingly, a forecast‐updating firm might procure earlier than a firm with no forecast updating. © 2009 Wiley Periodicals, Inc. Naval Research Logistics, 2009     

Impact of an all volunteer force upon the navy in the 1972-1973 timeframe

This paper develops estimates of true volunteer levels for 1972 and 1973, based on experience gained through 1970 draft lottery data. The paper also formulates estimates of the qualitative characteristics of a 1972-1973 Navy volunteer force, and establishes a relationship between rate of volunteerism and military pay. Utilizing estimates generated in the paper, Navy military personnel budget requirements for FY '72 and '73 are presented.     

Optimal design of a multi-item,multi-location,multi-repair type repair and supply system

The design of a system with many locations, each with many items which may fail while in use, is considered. When items fail, they require repair; the particular type of repair being governed by a probability distribution. As repairs may be lengthy, spares are kept on hand to replace failed items. System ineffectiveness is measured by expected weighted shortages over all items and locations, in steady state. This can be reduced by either having more spares or shorter expected repair times. Design consists of a provisioning of the number of spares for each item, by location; and specifying the expected repair times for each type of repair, by item and location. The optimal design minimizes expected shortages within a budget constraint, which covers both (i) procurement of spares and (ii) procurement of equipment and manning levels for the repair facilities. All costs are assumed to be separable so that a Lagrangian approach is fruitful, yielding an implementable algorithm with outputs useful for sensitivity analysis. A numerical example is presented.     

Competition under time‐varying demands and dynamic lot sizing costs

We develop a competitive pricing model which combines the complexity of time‐varying demand and cost functions and that of scale economies arising from dynamic lot sizing costs. Each firm can replenish inventory in each of the T periods into which the planning horizon is partitioned. Fixed as well as variable procurement costs are incurred for each procurement order, along with inventory carrying costs. Each firm adopts, at the beginning of the planning horizon, a (single) price to be employed throughout the horizon. On the basis of each period's system of demand equations, these prices determine a time series of demands for each firm, which needs to service them with an optimal corresponding dynamic lot sizing plan. We establish the existence of a price equilibrium and associated optimal dynamic lotsizing plans, under mild conditions. We also design efficient procedures to compute the equilibrium prices and dynamic lotsizing plans.© 2008 Wiley Periodicals, Inc. Naval Research Logistics 2009     

Newsvendor problems with sequentially revealed demand information

This article analyzes a capacity/inventory planning problem with a one‐time uncertain demand. There is a long procurement leadtime, but as some partial demand information is revealed, the firm is allowed to cancel some of the original capacity reservation at a certain fee or sell off some inventory at a lower price. The problem can be viewed as a generalization of the classic newsvendor problem and can be found in many applications. One key observation of the analysis is that the dynamic programming formulation of the problem is closely related to a recursion that arises in the study of a far more complex system, a series inventory system with stochastic demand over an infinite horizon. Using this equivalence, we characterize the optimal policy and assess the value of the additional demand information. We also extend the analysis to a richer model of information. Here, demand is driven by an underlying Markov process, representing economic conditions, weather, market competition, and other environmental factors. Interestingly, under this more general model, the connection to the series inventory system is different. © 2012 Wiley Periodicals, Inc. Naval Research Logistics 2012     

A priori error bounds for procurement commodity aggregation in logistics planning models

A complete logistical planning model of a firm or public system should include activities having to do with the procurement of supplies. Not infrequently, however, procurement aspects are difficult to model because of their relatively complex and evanescent nature. This raises the issue of how to build an overall logistics model in spite of such difficulties. This paper offers some suggestions toward this end which enable the procurement side of a model to be simplified via commodity aggregation in a “controlled” way, that is, in such a manner that the modeler can know and control in advance of solving his model how much loss of accuracy will be incurred for the solutions to the (aggregated) overall model.     

A nation-in-the-making,in arms: control of force,strategy and the Ukrainian Volunteer Battalions

At the core of waging war and strategy is the creation, control and use of force. This article investigates the volunteer battalions that mobilized in Ukraine during the spring of 2014. It contextualizes the volunteer phenomenon and focuses on the state strategies to establish control over these militias. As ambiguous entities arising from a situation characterized by rapid social change – revolution and war – the volunteer battalions threatened existing hierarchies and questioned state authority. The situation was exacerbated by the war, which deviated from the expectations of Ukrainian combatants and Western military observers alike. The state nevertheless enjoyed a modicum of success in reining in the militias through four strategies of undermining, co-option, incorporation and coercion. While predominantly integrated into a more rigid category of paramilitary forces, the volunteers continue to play a role in both the Ukrainian society and security sector to the unforeseeable future.     

基于VIKOR法的同类型装备集中采购评标模型研究

为了提高同类型装备集中采购效益,构建了同类型装备集中采购评标指标体系,综合运用熵权法和VIKOR方法,建立了同类型装备集中采购评标模型。应用实例结果表明,该模型可以用来对投标单位评估,并较精确地展现投标单位的技术和经济实力,为评标提供科学的依据。     

A deterministic inventory model for reparable items

A reparable inventory system has two distinct inventories within it—the inventory of items ready-for-issue and the inventory of carcasses available for repair. A reparable item is usually rebuilt upon failure, but the scrap rate in the repair process is generally positive. Consequently, new items must be procured from time to time to replace those item: which were scrapped. The ready-for-issue inventory has two input sources—procurement and repair, This paper develops a deterministic inbentory model for the reparable inventory system, and determines the optimal procurement and repair quantities.     

水下滑翔机建模与纵向运动控制

水下滑翔机是一种浮力驱动的新型水下航行器,其特殊的驱动方式和结构特点增加了建模和控制分析的复杂度。为获得准确有效的水下运动控制方法,通过滑翔机运动学和动力学分析,建立了较为完善的三维空间数学模型,并进行了合理的假设和简化,得到纵平面内小扰动线性化模型。在此基础上,运用了线性二次型最优控制理论设计了LQR调节器,分析系统对于控制指令的跟踪响应情况。最后针对滑翔机的纵平面运动进行了仿真分析,仿真结果显示,LQR调节器有很好的干扰抑制能力和指令跟踪性能,并准确地描述了纵平面内滑翔机的运动特性。     

The dynamic lot size model with quantity discount: Counterexamples and correction

Federgruen and Lee ([3]) proposed an optimal algorithm for the single-item dynamic lot size model with all-unit discount. In this note we show that their algorithm fails to find the optimal solution for some special cases. We also provide a modification to the algorithm to handle them. © 1998 John Wiley & Sons, Inc. Naval Research Logistics 45: 419–422, 1998     

European defence industry consolidation and domestic procurement bias

How have European cross-border defence industrial mergers and acquisitions affected domestic procurement bias among the major EU powers? This article departs from the findings of Andrew Moravcsik more than two decades ago suggesting that major West European states had no ingrained preferences for defence industrial autarchy. When cross-national armament projects were derailed, this could be attributed to political efforts of national defence industrial champions favouring purely domestic projects. As former national champions join pan-European defence groups, their preferences are likely modified. Does this shift procurement towards non-European “off-the-shelf” solutions which, according to Moravcsik, are favoured by defence departments? Or does it give impetus to a stronger preference for European as opposed to domestic systems? In this article, procurement patterns in the aftermath of cross-border defence industry consolidation will be analysed. Procurement bias is assessed in two industry segments characterised by pervasive consolidation.     

The dynamic transshipment problem

We investigate the strategy of transshipments in a dynamic deterministic demand environment over a finite planning horizon. This is the first time that transshipments are examined in a dynamic or deterministic setting. We consider a system of two locations which replenish their stock from a single supplier, and where transshipments between the locations are possible. Our model includes fixed (possibly joint) and variable replenishment costs, fixed and variable transshipment costs, as well as holding costs for each location and transshipment costs between locations. The problem is to determine how much to replenish and how much to transship each period; thus this work can be viewed as a synthesis of transshipment problems in a static stochastic setting and multilocation dynamic deterministic lot sizing problems. We provide interesting structural properties of optimal policies which enhance our understanding of the important issues which motivate transshipments and allow us to develop an efficient polynomial time algorithm for obtaining the optimal strategy. By exploring the reasons for using transshipments, we enable practitioners to envision the sources of savings from using this strategy and therefore motivate them to incorporate it into their replenishment strategies. © 2001 John Wiley & Sons, Inc. Naval Research Logistics 48:386–408, 2001     

A linear programming model for minimum-cost procurement and operation of marine corps training aircraft

In military situations of sharply increasing combat activity the Marine Corps is faced with training problems in its expanding aviator corps. Additional training aircraft are required, and procurement decisions must be made. In view of the significant costs involved in procurement and operation of new high performance aircraft, it is very desirable to buy and operate an efficient mix of aircraft necessary for training the pilots to make the Marine Aircraft Wings essentially 100-percent tactically qualified. The mathematical model presented here enables computation of a least-cost mix of training aircraft which satisfies certain specified training requirements. The basic element allowing tradeoffs is the commonality of training available in the F4, RF4, A6, and EA6 types of aircraft. Both airframe oriented and mission oriented training are necessary, but the airframe oriented training can be conducted in either of the aircraft possessing the commonality. Training requirements over a five year period are considered, and the mix of training aircraft has the minimum five year procurement and operating cost.     

An interactive optimization system for bulk-cargo ship scheduling

This article considers the efficient scheduling of a fleet of ships engaged in pickup and delivery of bulk cargoes. Our optimization system begins by generating a menu of candidate schedules for each ship. This menu can contain all feasible solutions, which guarantees we will find an optimal solution or can be heuristically limited to contain only those schedules likely to be in an optimal solution. The problem of choosing from this menu an optimal schedule for the fleet is formulated as a set-packing problem and solved with a dual algorithm. Computational experience is presented based on real data obtained from the Military Sealift Command of the U. S. Navy. Run times for this data were reasonable and solutions were generated with the potential of saving up to about $30 million per year over the manual system currently in place. We also describe a color-graphics interface developed to facilitate interaction with the optimization system.