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1、第三章 库存管理及风险共担 Inventory Management and Risk Pooling,供 应 链 管 理 Supply Chain Management,2,本章内容,单仓库库存实例 风险分担 集中式与分散化系统 供应链环境下的库存管理 实际问题 预测,经济批量模型 需求不确定性的影响 供应合同 多次订货 持续检查策略 可变提前期 定期检查策略,判断方法 市场研究方法 时间序列方法 因果方法 选择恰当的预测技术,3,Inventory(1),长期以来,库存管理的重要性、协调库存决策与运输策略的必要性已是显而易见 在复杂的供应链中,管理库存是相当困难且可能对顾客服务水平及供应链
2、整体系统成本有显著的影响 Where do we hold inventory? Suppliers and manufacturers Warehouses and distribution centres Retailers,4,Inventory(2),一个典型的供应链包含供货商、将原料转变成制成品的制造商,以及分销中心和将制成品分销给顾客的仓库 这隐含了库存在供应链中以数种形式存在: 原料库存(Raw Materials) 在制品库存(Work-in-Process,WIP) 制成品库存(Finished Goods),5,为何要持有库存,因应顾客需求的非预期变动 愈来愈多的产品拥有较
3、短的生命周期:表示和顾客需求有关的历史资料可能无法取得或受到相当的限制 市场上许多竞争产品的出现:产品产品的激增,使得对某一特定型号产品的需求预测变得更加困难 许多供应的数量和质量、供货商的成本和配送时间等出现明显的不确定情况 即使供应与需求皆无不确定性,由于运输提前期的关系,仍有必要持有库存 运输公司提供的规模经济,鼓励公司运输大量货品,因此也持有大量的库存,6,Goals: Reduce Cost, Improve Service(1),By effectively managing inventory: Xerox eliminated $700 million inventory fr
4、om its supply chain Wal-Mart became the largest retail company utilizing efficient inventory management GM has reduced parts inventory and transportation costs by 26% annually,7,Goals: Reduce Cost, Improve Service(2),By not managing inventory successfully In 1994, “IBM continues to struggle with sho
5、rtages in their ThinkPad line” (WSJ, Oct 7, 1994) In 1993, “Liz Claiborne said its unexpected earning decline is the consequence of higher than anticipated excess inventory” (WSJ, July 15, 1993) In 1993, “Dell Computers predicts a loss; Stock plunges. Dell acknowledged that the company was sharply o
6、ff in its forecast of demand, resulting in inventory write downs” (WSJ, August 1993),8,库存管理的两个重要议题,需求预测 订购量计算,9,影响库存策略的主要因素(1),第一且最重要的是顾客需求(Demand Characteristics) 它可能预先得知或具随机性 在后者之情况下,预测工具可以用于历史数据可取得的情况下,来估计平均顾客需求,以及顾客需求的变异性(通常以标准差衡量) 补货提前期(Lead Time) 可于订货时得知,但也可能是不确定的 仓库中不同产品储存的数目(Number of Produc
7、ts) 计划期的长度,10,影响库存策略的主要因素(2),成本(Costs),包含订购成本及库存持有成本 通常订购成本(Order Costs)包含二个组成元素:产品成本及运输成本 库存持有成本(Holding Costs)包含: 州税、财产税及库存保险(Taxes) 维修成本(Maintenance and Handling) 过时成本(Obsolescence),由于商品可能因市场改变而失去其部份价值的风险所造成 机会成本(Opportunity Costs):投资库存以外之其它标的物,可能获得的投资报酬(例如:股票市场) 要求的服务水平(Service Level) 在顾客需求不确定的情
8、况下,通常不可能每次都满足顾客的订购要求,所以管理当局必需具体指定一个可接受的服务水平,11,The Effect of Demand Uncertainty,Most companies treat the world as if it were predictable: Production and inventory planning are based on forecasts of demand made far in advance of the selling season Companies are aware of demand uncertainty when they c
9、reate a forecast, but they design their planning process as if the forecast truly represents reality Recent technological advances have increased the level of demand uncertainty: Short product life cycles Increasing product variety,12,Demand Forecast,The three principles of all forecasting technique
10、s Forecasting is always wrong(预测永远是错的) The longer the forecast horizon the worst is the forecast(预测期间越长,错误程度越高) Aggregate forecasts are more accurate(汇总预测更正确),13,Example : Swimsuit Production,14,Demand Scenarios,Average demand is about 13,000 units,15,Manufacturing Expected Profit,Manufacturing opti
11、mal production quantity is 12,000 units Manufacturer profit is $375,000,16,最适订购量不一定要等于预测或平均需求。事实上,最适订购量是根据多出售一单位之边际利润及边际成本间的关系来决定 当订购量增加,平均利润也会增加,直到生产数量达到某一个数值,在此数值之后,平均利润开始减少 当我们增加生产数量,风险(也就是产生大额损失的可能性)通常也会增加。同时产生大额利润的可能性也会增加,这就是风险/报酬间的互抵效果,Summaries,17,供应合同,为了确保足够的商品供应和需求,买方和供货商一般会签定供应合同,合同中可能规范以下
12、条款: 产品定价和数量折扣 最小与最大订购量 交货提前期 产品质量 产品退货策略,18,Wholesale Price =$80,Example : Swimsuit Production,19,Demand Scenarios,20,Distributor Expected Profit,$ 470,700,21,Supply Chain Expected Profit,Distributor optimal order quantity is 12,000 units Distributor expected profit is $470,700 Manufacturer profit i
13、s $440,000 Supply Chain Profit is $910,700,22,序贯优化(Sequential Optimization),序贯供应链:供应链中每个主体独立确定自己的行动,以追求自身利润的最佳化 如上述例子中,分销商作出采购决策以提高自己的利润,制造商再对该决策作出反应 序贯供应链不是一个有效的供应链伙伴战略 需要寻找“序贯优化” 转向“全局优化”的机制,以提高供应链各参与方的利润允许风险分担的各种供应合同,23,供应合同的种类(1),几种不同的供应合同可用来达到风险分担的效果,并增加供应链中各个成员的利润 回购合同(Buy-Back Contracts) 指卖方同
14、意以双方约定的价格向买方买回未售出的商品,24,回购合同 Retailer Profit (Buy Back=$55),25,回购合同 Manufacturer Profit (Buy Back=$55),回购合同之所以有效是因为它使制造商分担了分销商的部分风险,因而激励分销商增加订购数量,26,供应合同的种类(2),收益共享合同(Revenue-Sharing Contracts) 在序贯供应链中,分销商只订购一定数量产品的重要原因是批发价格较高 如果分销商可以说服制造商降低批发价,则分销商将会有增加订购量的动机 当然,如果不能售出更多数量的产品,降低批发价将会造成制造商利润的降低 在收益共
15、享合同下,买方让卖方分享其部分的收益,作为获得批发价折扣的回报,27,收益共享合同 Retailer Profit (Wholesale Price $60, RS 15%),28,收益共享合同 Manufacturer Profit (Wholesale Price $60, RS 15%),29,供应合同的种类(3),数量柔性合同(Quantity-Flexibility Contracts) 指供货商对未超过某一数量的退回(未售出)产品,提供全额的退款 和回购合同不同的是,数量柔性合同提供某一数量内的退货的全额退款,而回购合同则提供所有退货的部分退款 销售回扣合同(Sales Rebat
16、e Contracts) 当买方采购量超出某一特定数量时,每多采购一单位产品将获得供应商提供的一定回扣 销售回扣合同直接激励买方提高采购量,30,Supply Contracts,31,全局优化(Global Optimization) (1),问题:供应商和买方之间所期望的整体最大利润是多少? 若允许由一个理性的决策者确定整个供应链的最优策略,将会如何?,32,Wholesale Price =$80,Global Optimization:Example,33,Global Optimization:Supply Chain Profit,34,Comparison,但全局优化的困难在于,
17、需要将决策权交给一个理性的决策者,35,全局优化(Global Optimization) (2),供应合同通过允许买方与供货商共同承担风险和分享潜在收益,可帮助企业达到全局优化,而不需要借助一个理性决策者 若合理设计供应合同,就可获得与全局优化同样的利润 如泳装生产案例中若采用收益共享合同,当批发价降至75美元,而回购价升至65美元时,分销商愿意增加订购量到16000件,此时整个供应链的期望利润也为1014500美元,36,全局优化(Global Optimization) (3),从实施的观点看 全局优化只提供最优的信息,不提供在供应链伙伴之间分配利润的机制 供应合同不但提供了供应链伙伴之
18、间分配利润所采用的方式,而且使任一伙伴不可能通过偏离最优行动而获得更大利润,37,Supply Contracts: Case Study(1),Demand for a movie newly released video cassette typically starts high and decreases rapidly Peak demand last about 10 weeks Blockbuster purchases a copy from a studio for $65 and rent for $3 Hence, retailer must rent the tape
19、at least 22 times before earning profit Retailers cannot justify purchasing enough to cover the peak demand In 1998, 20% of surveyed customers reported that they could not rent the movie they wanted,38,Supply Contracts: Case Study(2),Starting in 1998 Blockbuster entered a revenue sharing agreement w
20、ith the major studios Studio charges $8 per copy Blockbuster pays 30-45% of its rental income Even if Blockbuster keeps only half of the rental income, the breakeven point is 6 rental per copy The impact of revenue sharing on Blockbuster was dramatic Rentals increased by 75% in test markets Market s
21、hare increased from 25% to 31%,39,多次订货(1),上一节有关泳装生产的案例中,假定在一个计划期内只有一次订货,这比较适合泳装、滑雪衫等销售季节很短的产品 但许多实际情况中,决策者可能在一年中任何时间重复订购产品 如考虑一个电视机分销商 分销商面对产品的随机需求,从制造商处获得产品供应 制造商无法实时满足分销商的订购 无论何时,当分销商下完订购单,运送都必须有一固定的提前期 因为需求是随机、无规则的,且制造商有一固定的运送提前期,即使订购产品不须支付固定的准备成本(Setup Cost),分销商仍需持有库存,40,多次订货(2),至少有三个理由使分销商要持有库存
22、: 满足提前期内发生的需求 应对需求的不确定性 平衡年库存持有成本及年固定订购成本 库存策略:决定何时订货以及订购多少 持续检查策略:每天检查库存水平,并作出是否订购及订购多少的决定 定期检查策略:每隔一固定期间检查一次库存水平,并确定合适的订购量,41,持续检查策略:假设,Normally distributed random demand:每日的需求是随机的,且服从正态分布 Fixed order cost plus a cost proportional to amount ordered:分销商每次向制造商订购产品,需要支付固定订购成本K,以及与订购数量成比例的费用 Inventory
23、 cost is charged per item per unit time:库存持有成本按每单位产品每单位时间来计算 If an order arrives and there is no inventory, the order is lost:假如接到顾客订购时,供货商手中没有库存(即缺货时) ,将失去此订单 The distributor has a required service level. This is expressed as the likelihood that the distributor will not stock out during lead time:分
24、销商规定一个要求的服务水平,即在提前期内不缺货的概率,42,持续检查策略,符号定义 库存状态(Inventory Position)的定义 在任何时间点的库存状态是仓库中实际库存加上分销商已订购但未送达的产品 (s, S) Policy: Whenever the inventory position drops below a certain level, s, we order to raise the inventory position to level S,43,持续检查之(s, S)库存策略,s:订货点(reorder point) S:最大库存水平(order-up-to lev
25、el),44,订货点(s),第一个是补货提前期内的平均库存,即平均每日需求及补货提前期的乘积: 确保直到下一次的订购送达前,将有足够的库存使用 第二个组成元素为安全库存,即分销商需在仓库及供应链渠道中维持的库存数量,来应对提前期期间内平均需求的误差: 订货点水平为:,45,最大库存水平(S),当库存状态下降到订货点水平s时,分销商发出批量为Q的订单。Q可根据经济订购批量公式计算: 最大库存水平为: 当使用(s, S)策略时,库存状态可能低于订货点,此时分销商就要订购足够多的数量,以使库存状态达到最大库存水平,显然这一订货量要大于Q。,46,平均库存水平,在两次相邻订货之间,最小库存水平是在收到
26、订货前的期望库存水平,即等于安全库存: 最大库存水平是订货到达之际的期望库存水平: 平均库存水平为上述两之和的平均:,47,Example,某电视机分销商试图对仓库中某型号的电视机制定库存策略。假定分销商发出订单需要支付一笔固定订货成本4500美元,分销商购买每台电视机的成本为250美元,平均库存持有成本为产品成本的18%,补货提前期为2周。下表提供了过去12个月的销售数据。分销商希望保证97%的服务水平,那么订货点水平和最大库存水平应该为多少?,48,Example (1),解:由上表可得平均月需求为191.17,月需求的标准差为66.53。 故平均周需求为191.17/4.3=44.58,
27、平均周需求的标准差为 提前期内的平均需求为44.582=89.16。由于服务水平为97%,查表可得安全系数为1.88,故安全库存为 订货点水平为:s=86.20+244.58176,49,Example (2),平均每周每台电视机的库存持有成本为: 因此订货批量为 最大库存水平为:S=Q+s=679+176=855 平均库存水平为:679/2+86.20426 这意味着分销商平均保持着约10周( 426/44.58)的库存。,50,可变提前期,在许多实际情况下,交货提前期固定且事先已知这一假设不一定成立,而是假设交货提前期服从正态分布,其平均提前期为AVGL,标准差为STDL 订货点水平为 最
28、大库存水平为,51,定期检查策略,在很多情况下,库存水平的检查是周期性的,且检查间隔期是固定的,每次检查后都要订购合适的数量 仓库先设定一个目标库存水平(即基本库存水平)及检查周期,再定期检查库存状态,并订购足够数量的货物,将库存状态提升到基本库存水平 符号定义,52,r:检查周期的时间长度 L:提前期,定期检查策略下的库存水平,53,基本库存水平,由于下一批订货在r+L天后到达,当前的订货要满足r+L天内的平均需求,即 (r+L)AVG 安全库存,即用来应对r+L天内平均需求的误差,计算公式为: 基本库存水平为:,54,平均库存水平,最大库存水平是订货到达之际的期望库存水平,即 最小库存水平
29、是在收到订货前的期望库存水平,即等于安全库存: 平均库存水平为:,55,Example,仍考虑前面的电视机分销案例,但假定分销商每3周发一次订单。 解:由于提前期是2周,因而基本库存水平要满足5周的需求。该时期的平均需求为44.585222.9 由于服务水平为97%,故安全库存为 基本库存水平为222.9+136.3 359 平均库存水平为,Service Level Optimization,Optimal inventory policy assumes a specific service level target. What is the appropriate level of se
30、rvice? May be determined by the downstream customer Retailer may require the supplier, to maintain a specific service level Supplier will use that target to manage its own inventory Facility may have the flexibility to choose the appropriate level of service,56,Service Level Optimization,FIGURE 2-11
31、: Service level inventory versus inventory level as a function of lead time,57,Trade-Offs,Everything else being equal: the higher the service level, the higher the inventory level. for the same inventory level, the longer the lead time to the facility, the lower the level of service provided by the
32、facility. the lower the inventory level, the higher the impact of a unit of inventory on service level and hence on expected profit,58,Retail Strategy,Given a target service level across all products determine service level for each SKU so as to maximize expected profit. Everything else being equal,
33、 service level will be higher for products with: high profit margin high volume low variability short lead time,59,Profit Optimization and Service Level,FIGURE 2-12: Service level optimization by SKU,60,Profit Optimization and Service Level,Target inventory level = 95% across all products. Service l
34、evel 99% for many products with high profit margin, high volume and low variability. Service level 95% for products with low profit margin, low volume and high variability.,61,62,风险共担(Risk Pooling),风险共担 假如我们把不同地点的需求汇总起来,需求变异性将会降低 因为当我们把不同地点的需求汇总时,来自某一位顾客的高需求将更可能被另一位顾客的低需求弥补掉 风险共担的三个重要观点 集中库存减少了分销系统中
35、的安全库存和平均库存 变异系数愈高,从集中式分销系统中所获得的利益愈大 来自风险共担的利益,依赖一市场区域和另一市场区域需求行为的相关性,63,集中式相对于分布式系统,在比较集中式和分布式分销系统时,我们必须了解两者间的权衡(Trade-Offs) 安全库存(Safety Stock):集中化安全库存会减少,减少的数量依赖于变异系数和不同市场之间的需求相关度。 服务水平(Service Level):当集中化和分散化具有同样的总安全库存时,集中化的服务水平相关较高。 间接成本(Overhead Costs):分散化系统管理费用较高。 顾客提前期(Customer Lead Time):分散化系
36、统响应时间较短。 运费(Transportation Cost),64,在供应链中管理库存,考虑一个多设施的供应链,并属于同一家公司 公司的目的在管理其库存以降低整体系统成本 因此考虑不同设施间的互动及这些互动对每个设施应采取库存策略的影响是很重要的 两项重要且合理的假设 库存决策由单一决策者制订,且这决策者的目标是使整体系统成本减到最小 决策者能够获取各分销商和仓库的库存信息,65,级库存,在这些假设下,库存策略是根据所谓级库存(Echelon Inventory)进行库存管理,是一个有效的库存管理系统 在分销系统中,每一阶段或层级(亦即仓库或分销商)被视为一级(Echelon)。因此,这系
37、统中任何阶段或层级的级库存等于此级中现有的库存,再加上所有的下游库存(Downstream Inventory) 例如,在仓库中的级库存等于在仓库的库存,再加上所有在途库存和各分销商储存的库存 仓库的级库存状态(Echelon Inventory Position)则是仓库中的级库存再加上那些仓库已订但尚未送达的商品,再减去缺货量,66,仓库的级库存,Reorder Point with Echelon Inventory,Le = echelon lead time, lead time between the retailer and the distributor plus the le
38、ad time between the distributor and its supplier, the wholesaler. AVG = average demand at the retailer STD = standard deviation of demand at the retailer Reorder point,67,4-Stage Supply Chain Example,Average weekly demand faced by the retailer is 45 Standard deviation of demand is 32 At each stage,
39、management is attempting to maintain a service level of 97% (z=1.88) Lead time between each of the stages, and between the manufacturer and its suppliers is 1 week,68,Costs and Order Quantities,69,Reorder Points at Each Stage,For the retailer, R=1*45+1.88*32*1 = 105 For the distributor, R=2*45+1.88*
40、32*2 = 175 For the wholesaler, R=3*45+1.88*32*3 = 239 For the manufacturer, R=4*45+1.88*32*4 = 300,70,More than One Facility at Each Stage,Follow the same approach Echelon inventory at the warehouse is the inventory at the warehouse, plus all of the inventory in transit to and in stock at each of th
41、e retailers. Similarly, the echelon inventory position at the warehouse is the echelon inventory at the warehouse, plus those items ordered by the warehouse that have not yet arrived minus all items that are backordered.,71,Inventory Positioning and Logistics Coordination,Multi-facility supply chain
42、 that belongs to a single firm Manage inventory so as to reduce system wide cost Consider the interaction of the various facilities and the impact of this interaction on the inventory policy of each facility Ways to manage: Wait for specific orders to arrive before starting to manufacture them make-
43、to-order facility Otherwise, decide on where to keep safety stock? Which facilities should produce to stock and which should produce to order?,72,Single Product, Single Facility Periodic Review Inventory Model,Assume - SI: amount of time between when an order is placed until the facility receives a
44、shipment (Incoming Service Time) S: Committed Service Time made by the facility to its own customers. T: Processing Time at the facility. Net Lead Time = SI + T - S Safety stock at the facility:,73,2-Stage System,Reducing committed service time from facility 2 to facility 1 impacts required inventor
45、y at both facilities Inventory at facility 1 is reduced Inventory at facility 2 is increased Overall objective is to choose: the committed service time at each facility the location and amount of inventory minimize total or system wide safety stock cost.,74,ElecComp Case,Large contract manufacturer
46、of circuit boards and other high tech parts. About 27,000 high value products with short life cycles Fierce competition = Low customer promise times Classic PUSH STRATEGY High shortages Huge risk PULL STRATEGY not feasible because of long lead times,75,New Supply Chain Strategy,OBJECTIVES: Reduce in
47、ventory and financial risks Provide customers with competitive response times. ACHIEVE THE FOLLOWING: Determining the optimal location of inventory across the various stages Calculating the optimal quantity of safety stock for each component at each stage Hybrid strategy of Push and Pull Push Stages
48、 produce to stock where the company keeps safety stock Pull stages keep no stock at all. Challenge: Identify the location where the strategy switched from Push-based to Pull-based Identify the Push-Pull boundary Benefits: For same lead times, safety stock reduced by 40 to 60% Company could cut lead
49、times to customers by 50% and still reduce safety stocks by 30%,76,Notations Used,FIGURE 3-11: How to read the diagrams,77,Trade-Offs,If Montgomery facility reduces committed lead time to 13 days assembly facility does not need any inventory of finished goods Any customer order will trigger an order for parts 2 and 3. Part 2 will be available immediately, since it is held in inventory Part 3 will be available in 15 days 13 days committed response time by the manufacturing facility 2 days transportation lead time. Another 15 days to process the order at the assembl
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