Forecasting lead times with economic data

Procurement lead time estimates are critical factors in production planning; however, they are generally based on a buyer's experience. The Westinghouse Corporation developed a linear model which successfully predicted procurement lead times for hot and cold rolled steel. This paper discusses nine models which use utilization and inventory data to predict lead time planning factors for aluminum, magnesium, steel and titanium products. The Lockheed-Georgia Company, which produces aerospace products, provided data for this research. The functional form of the models is significant and suggests that a nonlinear transformation of utilization based upon simple queueing models is a significant predictor of lead time duration.     

The optimal operation planning of a Kanban system with variable lead times

In the majority of the previous research on Kanban systems delivery lead time is treated as a fixed value. In many practical situations, however, the lead time is variable. In this paper, optimal operation planning of a fixed interval withdrawal Kanban with variable lead times is proposed. A cost is incurred for inventory level less than the safety inventory level, since the risk of shortage of inventory must be considered. Behaviour of the optimal number of Kanbans and the withdrawal interval of Kanbans are investigated in terms of various parameters such as standard deviation and mean lead time.     

Inventory control in hospital blood banks

The delicate relationship between donors and the Transfusion Service in the U.K. implies a clear duty to make the best possible use of the limited blood supplies, and this turns attention upon the stocking policies adopted by Hospital Blood Banks. Whereas Blood Bank staff can obtain an intuitive grasp of the effects of various stock levels for the common bloods, this is not usually the case for the rare bloods which are more difficult to control. A policy decision has to be made which balances the conflicting stock requirements of maintaining a high availability and also low out-dating rates. A Markovian structure is exploited in this paper for this purpose and the effects of the key variables are shown to be interrelated. In any particular instance the form of the probability distribution of demand between replenishments, the scale of the throughput, the blood life in relation to the ordering interval, the fraction of blood transfused and the fraction of the replenishment order which is supplied to the Bank will together determine the form of the operating characteristics.     

Inventory safety stocks of kanban control systems

Efficient production control systems should be flexible and lean. Pull control systems, especially kanban systems, lead to optimized organizational structures and order processes. This is due to their high transparency in practice. Up to now, their ability to adapt to dramatic changes in the order mix flexibility has been rated low. Existing kanban systems are based on non-adapting mathematical methods of buffer dimensioning and central organization. In this paper, the practical relevant methods of mathematical buffer dimensioning are described exemplarly. A new mathematical procedure is developed focusing on dynamic buffer stocks dimensioning including safety inventories. Its opportunities in practical employment are discussed. A realization of an electronic kanban system is described.     

The effect of MRP lot sizing on actual cumulative lead times in multi-level systems

This study investigates how different lot sizing techniques influence the cumulative lead time for multi-level production-inventory systems controlled by material requirements planning (MRP). Theoretical approaches, a numerical example, as well as simulation are used to analyse and illustrate the combined effect of lot sizing at different product structure levels. It is shown that lot-sizing requirements for more than a single period, such as fixed period requirements, period under quantity, Silver Meal algorithm, as well as economic order quantity will lead to longer actual cumulative lead times than would be expected, when taking the item lead times along the critical path through the product structure into account. Consequently, MRP will underestimate the cumulative lead time and will require a longer planning horizon. We show that the extension of the cumulative lead techniques covering the time is a lot-sizing related phenomenon and cannot be accounted for by, e.g. using safety lead time. Lot-sizing techniques with multi-period coverage will only occasionally provide the 'expected' cumulative lead time. We also show that average and maximum throughput times, as well as throughput time variability increases with increasing time-period coverage of lots.     

库存成本变化的VMI供应链协调

提出了库存成本变化的经济订货批量(EOQ)模型,基于该模型研究了库存成本变化时供应商管理库存(VMI)系统的最优协议问题。在该系统中,订货商和供应商达成缺货成本共担协议：当缺货发生时,供应商需要向订货商支付缺货补偿。订货商和供应商分散决策,订货商通过设计协议来减少其成本,而供应商通过制定补货决策来缩小自身成本。通过与传统系统和整合系统的比较,得出了库存成本变化时VMI系统的最优补货决策和缺货成本共担协议。采用数值算例验证了分析结果。结果表明,当且仅当供应商预期成本等于整合系统的最小总成本与固定缺货罚金之和时,VMI系统与整合系统具有相同的补货决策和系统绩效,即能够实现供应链协调。     

Inventory control of spares in a transport corporation — A case study

This paper reports on a study carried out in a transport organisation to find the optimal inventory of recoverable spares in a two-level inventory system consisting of a central depot and a set of 20 depots all over the territory of the organisation, with repair facilities. The problems of the present system are diagnosed and it is observed that the upper echelon's inability to meet the lower echelon's demand has resulted in an ever-increasing shortage. A mathematical model is developed to suit the system structure and a set of optimal inventory policies for recoverable spares at both the echelons is obtained for the case studied.     

Inventory control under different forms of uncertainty: Ambiguity and stochastic variability

Inventory decision makers routinely face ambiguity due to the psychological awareness that there is unknown information about salient events that is knowable in principle. Researchers on inventory control behavior in the face of uncertainty have primarily focused on uncertainty due to stochastic variability. However, most decision situations in the naturally occurring world involve both forms of uncertainty—ambiguity and stochastic variability. We report the results of two experiments that partial out the effects of ambiguity and stochastic variability by orthogonal manipulation of these two forms of uncertainty in a newsvendor task. Contrary to established mathematical models of decision making under uncertainty, increased ambiguity results in increased mean absolute percentage error, and a corresponding decrease in profit. We also find a systematic bias toward underordering associated with increased ambiguity, which is over and above the bias associated with increased stochastic variability. We do not see evidence for learning with repeated play, so that the effects of induced ambiguity appear to persist. Finally, based on our findings, we suggest measures that managers can use to ameliorate the effects of ambiguity.     

限额条件下的库存管理决策分析

在企业资金有限的情况下,常常会对总库存投资水平设定限额.采购经理和库存管理人员就不能完全依照经济订货批量(EOQ)基本模型来安排采购与库存管理了,在应用模型计算的结果作为决策依据之前,必须在新的条件下采取适当的措施对EOQ基本模型加以修正.     

The impact of stochastic lead times on the bullwhip effect under correlated demand and moving average forecasts

We quantify the bullwhip effect (which measures how the variance of replenishment orders is amplified as the orders move up the supply chain) when both random demands and random lead times are estimated using the industrially popular moving average forecasting method. We assume that the lead times constitute a sequence of independent identically distributed random variables and the correlated demands are described by a first-order autoregressive process. We obtain an expression that reveals the impact of demand and lead time forecasting on the bullwhip effect. We draw a number of conclusions on the bullwhip behaviour with respect to the demand auto-correlation and the number of past lead times and demands used in the forecasts. We find maxima and minima in the bullwhip measure as a function of the demand auto-correlation.     

Inventory control for the supply chain: An adaptive control approach based on the identification of the lead-time

In this paper, an Internal Model Control (IMC) scheme is incorporated in production inventory control systems in a complete supply chain. This control scheme presents a good target inventory tracking under the perfect knowledge of the system. Furthermore, the inventory tracking and load disturbance rejection control problems can be tackled separately. However, the closed-loop performance of the IMC scheme may be degraded due to a mismatch between the modelled and actual delay or to the fact that delays may be time-varying. Thus, the IMC control scheme is enhanced in this work with a novel method for the online identification of lead times based on a multimodel scheme. In this way, all benefits of the IMC scheme can be exploited. A detailed discussion of the proposed production inventory system is provided including a stability and performance analysis as well as the identification capabilities of the algorithm. Several simulation examples illustrate the efficiency of the approach.     

应对突发事件的库存共享策略

以包含两个销售商的库存系统为例,构建了一个描述库存共享应对突发事件的非合作博弈模型,证明了其纳什均衡解是唯一存在的。研究结果显示库存共享策略总有可能使得销售商的期望利润得以改进。另外,比较静态分析反映了转载价格和转载成本显著影响库存共享销售商的最优订货量和最优期望利润。最后,提出了一个简单的求解模型纳什均衡解的启发式算法。     

The importance of lead time control in the order fulfilment process

This paper reports a study of the order fulfilment process, with a particular emphasis on internal and external lead times, in a speciality chemical manufacturer. Amongst the main findings are that the company has inadequate control of its external lead times. For example, it was discovered that there is no policy on lead times, lead timesare rarely negotiated, lead times are only reviewed once per year and when a supplier gives a flexible lead time it is always the longest one which is entered into the system. It is also pointed out that internal lead timesare affected by a number of inefficient departmental practices, amongst these are those found in the finished goods warehouse, i.e. order picking, product rotation, product shelf life, working environment, communication and performance monitoring.     

含有自治复原退货物流的单周期库存问题研究

研究电子商务模式中含有自治复原退货物流的单周期库存控制问题.在考虑退货价对需求影响的基础上,通过引入可再售退货产品率,将退货品分为可再销售和不可再销售两部分进行分析,以销售商的利润最大化为目标,借助报童模型建立了自治复原退货物流下考虑退货价对需求影响的单周期库存模型,分析模型中最优订货量、退货价和利润的求解方法,通过数值分析探讨模型中各参数(退货价对需求的影响因子、退货率、可再售退货率)对利润、最优退货价和订购量大小的影响以及造成这些影响的原因.研究结果发现,商家应该在制定策略时考虑退货价对需求的影响,并大胆地提出和宣传自己的退货政策,以增加顾客对退货价的敏感度,从而获取更多的利润.     

与时俱进 争创一流

大涧村位于禹州市西部,素有九龙朝风贵地之美称,总面积4.5平方公里,1800多人,村党委下设五个党支部,115名党员。这里地下矿产资源丰富,经济发展势头强劲,境内有十几种优质煤及硫矿石、铜矿石、银     

Tackling seasonality : The case of Sri Lanka

Seasonality can be analysed at three levels — national, regional, and sectoral. In Sri Lanka there is a direct relationship between the product/ market mix of individual regions and their seasonality. The south-west coast resorts, with their November–March high season, are a major factor in the national seasonal pattern. Several diversification measures are considered and their probable impact assessed. A distinction is made between seasonal markets(eg Sweden) and counter-seasonal markets (eg India), and between inherent and expedient seasonality. The author discusses the deliberate manipulation of the supply of accomodation and its main effects.     

Online scheduling on a single machine with linear deteriorating processing times and delivery times

Journal of Combinatorial Optimization - This paper considers a class of problems with linear deteriorating jobs. Jobs are released over time and become known to the online scheduler until their...     

基于提前期的多阶通用件库存模型

针对单个企业内部应用通用件的环境,在假定一个多阶生产系统的基础上,对应用通用件的库存量水平及其成本进行了分析,建立了多阶通用件库存模型,提出通过对采用通用件和不采用通用件的成本差异的比较,来决策多阶生产系统中在哪一阶工序采用通用件,并求解出每一阶工序优化的基本库存水平。