库存管理讲义

2009/10/281Unit 4 & 5Inventory Management(I & II)2009/10/282Outline|Functions of Inventories|Types of Inventory|Cost Elements in Inventory|Economic Order Quantity|continuous review & periodic reviewpolicies|ABC Analysis|Effective Inventory Management2009/10/283Inventory is an important performancemeasure for a supply chain|In 1998, Dell had 10 days of inventoryand Compaq had 100 days.|When the prices of PC dropped,Compaq was hurt the most ………2009/10/284What is Inventory|Inventory are materials and supplies thata business or institution carries either forsales or to provide inputs or supplies tothe production process. All business andinstitutions require inventories.|they are a substantial part of total assets.2009/10/285Inventory Management|Inventory Management is responsible for planning and controlling inventory from raw material stage to the customer. Since inventory either results from production or supports it, the two cannot be managed separately and, therefore, must be coordinated |Inventory must be considered at each of theplanning levels and is thus part of productionplanning, master production scheduling, andmaterial requirement planning.2009/10/286Role of inventory in the supplychain|Stabilizes the production andworkforce|Supply/Demand uncertainty|Material delivery lead time uncertainty|Speculation (value of item expected toincrease in the future)|Economies of scale in purchasing andtransportation2009/10/287Why Inventory ExistsRate of supply from input processInventoryRate of demandfrom output process 2009/10/288Functions of Inventories|Inventory serves as buffer between:z Supply and demandz Customer demand and finished goodsz Finished goods and componentavailabilityz Requirements for an operation and theoutput from the preceding operationz Parts and materials to begin productionand the suppliers of materials2009/10/289Factors affecting inventorylevel|There are many factors affecting theinventory level and hence the inventory costin a Supply Chain, the following are some ofthe most important ones:zService level zDemand and supply variability zLead time and lead time variability zInformation (demand information)z Supply chain cost2009/10/2810Type of Inventory|Anticipation Inventory|Buffer Inventory|Cycle Inventory|Pipeline Inventory2009/10/2811Anticipation Inventory|Anticipation inventories are built up inanticipation of future demand.|For example, they are created ahead of apeak selling season, a promotion program,vacation shutdown, or possibly the threat ofa strike.|They are built up to help level productionand to reduce the costs of changing production rates 2009/10/2812Buffer Inventory (Safety Stock)|Inventory is held to cover randomunpredictable fluctuations in supply anddemand or lead time.|If demand or lead time is greater thanforecast, a stockout will occur.|Safety stock is carried to protect againstthis possibility. Its purpose is to preventdisruptions in manufacturing or deliveries tocustomers.|Safety stock is also called reserve stock.2009/10/2813Cycle Inventory|Cycle inventory occurs becauseone or more stages in the operationcannot supply all the items itproduces simultaneously.2009/10/2814Pipeline Inventory|Pipeline inventory exist because of the timeneeded to move goods from one location toanother such as from a plant to a distributioncentre of a customer.|They are sometimes called TransportationInventory or Movement Inventory.2009/10/2815Objectives of Inventory Management|A firm wishing to maximize profit willhave at least the following objectives:•Maximum customer service•Low-cost plant operation•Minimum inventory investment 2009/10/2816Possible InventoryLocationSuppliers CustomerTotal company InventoryManufacturingDistributionCustomer Delivery Raw Materials Inventory Work-in-process Inventory Finished Product Inventory2009/10/2817Cost Elements in Inventory1.Item cost 2.Carrying costs 3.Ordering costs 4.Stockout costs 5.Capacity-associated costs 2009/10/2818Item Cost|Item cost is the price paid for a purchase item, which consists of the cost of the item and any other direct costs associated in getting the item into the plant. |These could include such things as transportation, custom duties, and insurance. The inclusive cost is often called the landed price .|For an item manufactured in-house, the cost includes direct material, direct labor, and factory overhead . |These costs can usually be obtained from eitherpurchasing or accounting.2009/10/2819Carrying Costs|Carrying Costs include all expenses incurred by the firm because of the volume of inventory carried. |As inventory increase, so do thesecosts. They can be broken downinto three categories:1.Capital costs2.Storage costs3.Risk costs 2009/10/2820Ordering Costs|Every time an order is placed to replenish stock a number of transactions are needed which incur costs to the company. |These include the clerical tasks ofpreparing the order and all thedocumentation associated with it,arranging for the delivery to be made,arranging to pay the supplier for thedelivery, and the general costs ofkeeping all the information whichallows us to do this.2009/10/2821Stockout Costs|If demand during the lead time exceeds forecast, we can expect a stockout. |A stockout can potentially be expensive because of back-order costs, lost sales, and possibly lost customers.|Stockout can be reduced by carrying extra inventory to protect against those times when the demand during lead time is greater than forecast.2009/10/2822Capacity -Associated Costs|When output levels must be changed, there may be costs for overtime, hiring, training, extra shift, and layoffs.|These capacity-associated costs can be avoided by leveling production,that is, by producing items in slack periods for sale in peak periods.|However, this builds inventory in the slack periods.2009/10/2823Cost trade-offs to Be Considered by the Purchasing Executive* The savings associated with volume buying include:Lower per-unit purchasing prices Lower transportation costsLower warehouse handling costs Lower order-processing costsLower production lot quantity costs Lower stockout costsThe costs of carrying inventory include:Capital costs associated with the inventory investment Inventory service costs (insurance and taxes)Storage space costs Inventory risk costs Cost savings associated with volume buying *Inventory carrying costs ±Source: Douglas M. Lambert and Jay U. Sterling, “Measuring Purchasing Performance,”Production and InventoryManagement Review 4, no.6 (June 1984), p. 52. Reprinted with permission from P&IM Review, June 1984. Copyright 1984 by T.D.A. Publications, Inc., Hollywood, FL.2009/10/2824Methods of Accounting for Inventory|First-in, First-out (FIFO)zStock acquired earliest is assumed to be sold first, leaving stock acquired more recently in inventory |Last-in, First-out (LIFO)zSales are made from the most recently acquired stock, leaving items acquired in the earliest time period in inventory|Average CostzEach new purchase is averaged with the remaining inventory to obtain a new average price2009/10/2825Class Exercise2009/10/2826Inventory Cycle ChartPeriods of Time (days)Q u a n t i t y o f I t e m i n S t o c kSafety LevelZero StockOperating LevelPeak inventoryOrder Lead TimeRe-Order CycleReceipt of supplyOrder PointConsumption ReplenishmentAverage Inventory2009/10/2827Economic Order Quantity Method|The EOQ method is an attempt to estimate the best order quantity by balancing the conflicting cost of holding stock and of placing replenishment orders|The large order quantity gives a much higher average stock level , and the small order quantity gives a lower average stock level |The small order quantity must bereplenished by placing many more orders than the large order quantity2009/10/2828Economic Order Quantity MethodAssumptions1.Demand is relatively constant and is known.2.The item is produced in lots or batches and not continuously3.Order preparation and inventory-carrying costs are constant and known.4.Replacement occurs all at once.2009/10/2829The Economic Order QuantityI n v e n t o r y L e v e lTimeDemand (D) = 1000 items per yearPlan A Q = 400Plan B Q = 100Average Inventory for Plan A = 200Average Inventory For Plan B = 500.1 Year0.4 Year2009/10/2830The Economic Order Quantity (2)|Average Inventory = Order Quantity/2zPlan A = 200z Plan B = 50|Number of Orders per year = Annual Demand/Order QuantityzPlan A = 1000/400 = 2.5z Plan B = 1000/100 = 10|Annual Cost of Placing Orders |Annual Cost of Carrying Inventory2009/10/2831The Economic Order Quantity (3)D =Annual Usage in Units (1000)S =Ordering Cost in Dollars Per Order ($20)i =Annual Carrying Cost rate as a % (50%)c =Unit Cost in Dollars ($2)Q=Order Quantity in Units (400, 100)|Annual Cost of Placing Ordersz Plan A (2.5 x 20 = $50)zPlan B (10 x 20 = $200)|Annual Cost of Carrying Inventoryz Plan A (400/2 x $2 x 50% = $200)zPlan B (100/2 x $2 x 50% = $50)2009/10/2832Cost & Ordered Quantity Trade-offTotal CostsCarrying CostsProcurement (ordering cost) & Out-of-stock CostsEOQQuantity OrderedT o t a l R e l e v a n t C o s t s2009/10/2833|The optimum quantity for inventory procurement when procurement cost equals to carry costEconomic Order Quantity2DS ICEOQ =D = Annual Usage in UnitsS = Ordering Cost in Dollars Per OrderI = Annual Carrying Cost rate as a % C = Unit Cost in Dollars Q = Order Quantity in Units2009/10/2834Example of EOQAnnual Demand (D): 1000 units Ordering Cost (S):$20Carrying Cost Rate (i):50%Unit Cost in $ (c):$2Q = 2(1000)(20)$2 x 0.5= 400001EOQ = 200 units2009/10/2835Example Problem|An item has an annual demand of 5000 units, Ordering costs are $20 per order, and the cost of carrying inventory is 20%. The cost per unit is $5.|What is:z D = z S = z c = zi = |Then :zEOQ =2009/10/2836Example Problem|An item has an annual demand of 5000 units, Ordering costs are $20 per order, and the cost of carrying inventory is 20%. The cost per unit is $5.|What is:z D = 5000 units z S = $20z c = $5zi = 0.2|Then :zEOQ = 2 x 5000 x 205 x 0.2= 447.21 or 448 units2009/10/2837Class exercise2009/10/2838Fixed order quantity system|Perpetual inventory system|Event triggered: Initiates order when stock depleted to a specific level.zReorder point|Inventory replaced in fixed amountszEconomic order quantities|Issues: visual signals, IT applications2009/10/2839Fixed Order Quantity Systemlead time (L)ROPcycle stockINVENTORYTIMEROP = Lead time demand2009/10/2840Safety Stock|Safety stock is held because of uncertainty in supply and/or demand|The trade-off is the cost of stocking out versus the cost of holding inventory|Safety stock levels can be calculated using statistical techniques.z e.g., Take into account standard deviation of demand2009/10/2841Fixed Order Quantity System:Cycle Stock, Safety Stock and Lead Timelead timeROPcycle stock (Q)INVENTORYTIMESafety Stock2009/10/2842Fixed time period systems|Inventory on-hand counted at specific time intervals and replenished to a desired level|Only the passage of time triggers the model2009/10/2843Fixed Time Period System :Cycle Stock, Safety Stock and Lead TimeINVENTORYTIMESafety Stockreview periodlead timeQ2009/10/2844Replenishment Policies|When to reorder?|How much to reorder?|Continuous Review : order fixed quantity when total inventory drops below reorder point (ROP)|Periodic review : order at fixed time intervals to raise total inventory to order up to level (OUL)2009/10/2845Inventory systems|Continuous Review system:zsystem that keeps track of removals from inventory continuously, thusmonitoring current levels of each item. z When stock level reaches a certain level (ROP), an order of fixed quantity (EOQ) is placed2009/10/2846Inventory systems|Periodic review system:zOrders are placed at fixed review period (RP) intervals.z The order quantity varies and is calculated to meet somepredetermined target inventory level (TIL)2009/10/2847Comparing continuous review & periodic review policies|Factors driving safety inventory in periodic review policyz Demand uncertaintyz Replenishment lead time z Service level zReorder interval|Periodic review policy is easier and cheaper to implement|Periodic review policy requires more safety inventory than continuous review policy for the same lead time and service level2009/10/2848Inventory Priorities –ABC SystemPercentage Percentage value of itemsof annual usage Class A items About 20%About 80%Class B items About 30%About 15%Class C itemsAbout 50%About 5%2009/10/2849Classifying Inventory|ABC Analysis based on Pareto ’s Law –the “80 –20 Rule ”.|Decision steps in applying ABC Analysis:zSelect criterion appropriate for inventory under consideration eg sales revenue z Rank items in descending order of importancez Calculate actual and cumulative total sales revenuez Assign items into ABC groups 2009/10/2850Product Classification Analysis (ABC)Product Number Sales (000)Percent of Sales CumulatoveSales PercentCumulatirveProducts PercentCategory145,00030.030.05A 235,00023.353.310A 325,00016.770.015A 415,00010.080.020A 58,000 5.385.325B 65,000 3.388.730B 74,000 2.791.335B 83,000 2.093.340B 92,000 1.394.745B 101,0000.795.350B 111,0000.796.055C 121,0000.796.760C 202500.2100.00100C2009/10/2851A Typical Pareto Curve with ABC AnalysisS a l e s V a l u eNumber of Products20%50%100%80%95%100%Class CClass BClass A2009/10/2852Steps in Making an ABC Analysis1.Establish the item characteristics that influence the results of inventory management. This is usually annual dollar usage but may be other criteria, such as scarcity of material.2.Classify items into groups based on the established criteria.3.Apply a degree of control in proportion to the importance of the group.2009/10/2853Control Based on ABC Classification|Different controls used with different classifications might be the following:z A items: high priority.•Tight control including complete accurate records, regular and frequent review by management, frequent review of demand forecasts, and close follow-up and expediting to reduce lead timezB items: medium priority.•Normal controls with good records, regular attention, and normal processing.zC items: lowest priority.•Simplest possible controls –make sure there are plenty.•Simple or no records; perhaps use a two-bin system or periodic review system.•Order large quantities and carry safety stock .2009/10/2854ABC AnalysisA Items|Very tight control|Order only calculated or know requirement |Accurate recording of receipts and issues |Schedules constantly reviewed |Continuous progressing|Minimal buffer stocks (probably less than 2 weeks)2009/10/2855B Items|Moderate level of control|Order against forecast from historical data |Recording of all receipts and issues |Moderate level of review of schedules |Progress items in short supply or late |Larger buffer stocks (6 –8 weeks)ABC Analysis (Cont)2009/10/2856ABC Analysis ( Cont )C Items|Lower level of control|Minimal recording of receipts/issues |Low level of schedule review |No progressing|Large safety stocks (12 weeks)2009/10/2857Class Exercise2009/10/2858Symptoms of Poor Inventory|Increasing numbers of back orders|Increasing dollar investment in inventory with back orders remaining constant.|High customer turnover rate.|Increasing number of orders being cancelled.|Periodic lack of sufficient storage space.|Wide variance in inventory turnover among distribution centers and major inventory items.Effective Inventory Management|Traditional method –adequate safetystock of inventory|Contemporary method –adoption of VMI & ECR|EIM depends highly on IT communicationnetwork require information sharingbetween suppliers and customers shiftingresponsibility for managing andreplenishing inventory to vendor2009/10/28 59。

第12章库存管理库存管理是生产管理体系中最为悠久的分支之一，库存管理的内容包含仓库管理和库存控制两个部分。

仓库管理的内容是指库存物料的科学保管，以减少损耗，方便存取；库存控制则是要求控制合理的库存水平，即用最少的投资和最少的库存管理费用，维持合理的库存，以满足使用部门的需求和减少缺货损失。

本章主要讨论库存控制。

库存管理概述库存(Inventory )的定义国民经济的各个行业，尤其是制造业和服务业都会遇到库存问题。

狭义的理解，库存即放在仓库中，暂时未被利用的物资。

从广义上理解，凡是处于暂时闲置状态，尚未被利用的各类社会资源都可被视为库存。

与这种资源是否存放在仓库中，是否处于运动状态没有关系。

放在仓库里是闲置，准备被利用，铁路运输中的货物，是为了未来需要而闲置在途中，是一种在途库存。

库存的作用库存是闲置的资源，不能立即为企业产生效益，但库存又是必须的，因为库存有以下重要的作用。

1•满足预期顾客的需求一定的成品库存可以使顾客很快采购到他们所需要的物品。

这样，可以缩短顾客的订货提前期，也有利于供应厂商争取预期顾客。

2.平滑生产的均衡性外部的需求总是波动的，而企业的生产要求具有均衡性。

要满足需方的波动的需求，又要使供方的生产均衡，就必须维持一定量的库存。

3.分摊订货费用订货需要费用，需要一件订购一件，将订货费用摊在一件物品上，是不经济的。

一次采购一批，虽然会造成库存，但可以将订货费用分摊到各件物品上，是经济的。

在生产过程中，采用批量生产加工，可以分摊生产准备费用和结束整理费用。

4.防止缺货维持一定量的库存，可以防止缺货的产生。

商店没有一定量的货物库存，顾客就买不到东西；酒店没有一定的床位库存，游客就不能入住；在生产过程中维持一定量的在制品，可以防止生产因缺货而中断。

5. 避免价格上涨企业对有涨价可能性的物资会加大库存量，也会通过加大订货量以获取数量折扣。

库存具有重要的作用，但也有其不利的一面。

库存要占用资金，物资的库存要修建仓库；要维持库存物品不损耗、不老化，都需要大量支出。