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1、OPERATIONS MANAGEMENT Chapter 12 Inventory Management中南财经政法大学 工商管理学院 刘大明OPERATIONS MANAGEMENTChapter 12Inventory Management*中南财经政法大学 工商管理学院 刘大明Amazon4Amazon started as a “virtual” retailer no inventory, no warehouses, no overhead; just computers taking orders to be filled by othersGrowth has forced
2、Amazon to become a world leader in warehousing and inventory management中南财经政法大学 工商管理学院 刘大明*中南财经政法大学 工商管理学院 刘大明AmazonEach order is assigned by computer to the closest distribution center that has the product(s)A “flow meister” at each distribution center assigns work crewsLights indicate products tha
3、t are to be picked and the light is reset (pick-to-light)Items are placed in crates on a conveyor. Bar code scanners scan each item 15 times to virtually eliminate errors.中南财经政法大学 工商管理学院 刘大明*中南财经政法大学 工商管理学院 刘大明AmazonCrates arrive at central point where items are boxed and labeled with new bar codeGi
4、ft wrapping is done by hand at 30 packages per hourCompleted boxes are packed, taped, weighed and labeled before leaving warehouse in a truckOrder arrives at customer within a week中南财经政法大学 工商管理学院 刘大明*中南财经政法大学 工商管理学院 刘大明AmazonA “flow meister” at each distribution center assigns work crews中南财经政法大学 工商管
5、理学院 刘大明*中南财经政法大学 工商管理学院 刘大明AmazonLights indicate products that are to be picked and the light is reset (“pick-to-light” system)中南财经政法大学 工商管理学院 刘大明*中南财经政法大学 工商管理学院 刘大明AmazonItems are placed in crates on a conveyor. Bar code scanners scan each item 15 times to virtually eliminate errors.中南财经政法大学 工商管理学
6、院 刘大明*中南财经政法大学 工商管理学院 刘大明AmazonGift wrapping is done by hand at 30 packages per hour中南财经政法大学 工商管理学院 刘大明*中南财经政法大学 工商管理学院 刘大明AmazonCompleted boxes are packed, taped, weighed and labeled before leaving warehouse in a truck中南财经政法大学 工商管理学院 刘大明*中南财经政法大学 工商管理学院 刘大明InventoryOne of the most expensive assets
7、of many companies representing as much as 50% of total invested capitalOperations managers must balance inventory investment and customer service中南财经政法大学 工商管理学院 刘大明*中南财经政法大学 工商管理学院 刘大明Types of InventoryRaw materialPurchased but not processedWork-in-processUndergone some change but not completedA fun
8、ction of cycle time for a productFinished goodsCompleted product awaiting shipmentMaintenance/repair/operating (MRO)Necessary to keep machinery and processes productive中南财经政法大学 工商管理学院 刘大明*中南财经政法大学 工商管理学院 刘大明Functions of InventoryTo ”decouple” or separate various parts of the production processTo pro
9、vide a stock of goods that will provide a “selection” for customersTo take advantage of quantity discountsTo hedge against inflation and upward price changesBuffer InventorySafety InventoryCycle InventoryPipeline InventorySeasonal Inventory中南财经政法大学 工商管理学院 刘大明*中南财经政法大学 工商管理学院 刘大明The Material Flow Cyc
10、leFigure 12.1Pipeline Inventory中南财经政法大学 工商管理学院 刘大明*中南财经政法大学 工商管理学院 刘大明Higher costsItem cost (if purchased)Ordering (or setup) costCosts of forms, clerks wages etc.Holding (or carrying) costBuilding lease, insurance, taxes etc.Difficult to controlHides production problemsDisadvantages of Inventory中南财
11、经政法大学 工商管理学院 刘大明*Of the items listed on this slide, the least obvious to most students is the manner in which inventory can be used to hide production problems.*中南财经政法大学 工商管理学院 刘大明Divides inventory into three classes based on annual dollar volumeClass A - high annual dollar volumeClass B - medium an
12、nual dollar volumeClass C - low annual dollar volume $ volume = Annual demand x Unit costUsed to establish policies that focus on the few critical parts and not the many trivial onesDevelop class A suppliers moreGive tighter physical control of A itemsForecast A items more carefullyABC Analysis中南财经政
13、法大学 工商管理学院 刘大明*It might be helpful here to discuss some of the differences in the ways we would manage items in the three different levels. What actions would we actually take in managing A versus in managing C?*中南财经政法大学 工商管理学院 刘大明Classifying Items as ABCFigure 12.2中南财经政法大学 工商管理学院 刘大明*Are we back to
14、 Pareto analysis?*中南财经政法大学 工商管理学院 刘大明Example 1中南财经政法大学 工商管理学院 刘大明Sheet1ItemAnnual usageUnit costAnnual dollar volumePercentAccumulative percentPercent of itemsClass#10286100090.0090000.0038.80A#11526500154.0077000.0033.2072%20%A#12760155017.0026350.0011.40B#1086735042.8615001.006.50B#10500100015.501
15、2500.005.4023%30%B#1257260014.178502.003.70C#1407520000.601200.000.50C#010361008.50850.000.40C#0103712000.42504.000.20C#105722500.60150.000.105%50%C合计8550232057.00100.00Sheet2Sheet3*中南财经政法大学 工商管理学院 刘大明Independent versus Dependent DemandIndependent demand - demand for item is independent of demand fo
16、r any other itemDependent demand - demand for item is dependent upon the demand for some other item中南财经政法大学 工商管理学院 刘大明*中南财经政法大学 工商管理学院 刘大明Inventory CostsHolding costs - associated with holding or “carrying” inventory over timeOrdering costs - associated with costs of placing order and receiving good
17、sSetup costs - cost to prepare a machine or process for manufacturing an order中南财经政法大学 工商管理学院 刘大明*中南财经政法大学 工商管理学院 刘大明Holding CostsTable 12.1CategoryCost (and Range) as a Percent of Inventory ValueHousing costs (including rent or depreciation, operating costs, taxes, insurance)6% (3 - 10%)Material ha
18、ndling costs (equipment lease or depreciation, power, operating cost)3% (1 - 3.5%)Labor cost3% (3 - 5%)Investment costs (borrowing costs, taxes, and insurance on inventory)11% (6 - 24%)Pilferage, space, and obsolescence3% (2 - 5%)Overall carrying cost26%中南财经政法大学 工商管理学院 刘大明*中南财经政法大学 工商管理学院 刘大明Determi
19、nant modelsBasic economic order quantityProduction order quantityQuantity discountProbabilistic modelsFixed order-quantity modelsFixed order-period modelsdetermine when and how much to order.? 1984-1994 T/Maker Co.Inventory Models for Independent Demand中南财经政法大学 工商管理学院 刘大明*This slide simply introduce
20、s some of the available models. Additional details are provided in subsequent slides.*中南财经政法大学 工商管理学院 刘大明Demand is known, constant, and independentLead time is known and constantReceipt of inventory is instantaneous and completeQuantity discounts are not possibleOnly variable costs are setup and hol
21、dingStockouts can be completely avoidedEOQ Assumptions中南财经政法大学 工商管理学院 刘大明*Students should be asked to consider the degree to which each of these assumptions is accurate.*中南财经政法大学 工商管理学院 刘大明EOQ ModelFigure 12.3中南财经政法大学 工商管理学院 刘大明*One should link this model to the assumptions. You should also explore,
22、 at least briefly, how this picture would change if the assumptions were not met.*中南财经政法大学 工商管理学院 刘大明Deriving an EOQ中南财经政法大学 工商管理学院 刘大明*Students may find it helpful if you actually go through each of these steps - at least through writing the equation, and setting setup cost equal to holding cost.*中
23、南财经政法大学 工商管理学院 刘大明How Much to Order?Table 11.5The very shallow curve中南财经政法大学 工商管理学院 刘大明*中南财经政法大学 工商管理学院 刘大明Example 3,4,5D=1000 unitsS=$10 per orderH=$0.50 per unit per yeard=1000 / 250 = 4 units per day (250 days per year)L=5 days中南财经政法大学 工商管理学院 刘大明*中南财经政法大学 工商管理学院 刘大明Robust Model ExampleEOQ increas
24、es by only 23%Total annual cost increases by only 23%中南财经政法大学 工商管理学院 刘大明*中南财经政法大学 工商管理学院 刘大明Robust ModelThe EOQ model gives satisfactory answers even with substantial variation in its parameters.When its some parameter is changed by r (eg. D0=D(1+r)The total cost curve is relatively flat in the area
25、 of the EOQ中南财经政法大学 工商管理学院 刘大明*中南财经政法大学 工商管理学院 刘大明Robust Model ExampleTotal annual cost increases by only 8%EOQ increases by only 50%D = 1,000 units S = $10 per orderH = $0.50 per unit per year中南财经政法大学 工商管理学院 刘大明*中南财经政法大学 工商管理学院 刘大明Robust ModelThe cost of the EOQ changes little in the neighborhood o
26、f the minimum.When Q* is changed by (|<1), (eg. Q0=Q(1+)中南财经政法大学 工商管理学院 刘大明*中南财经政法大学 工商管理学院 刘大明Robust Model: , 中南财经政法大学 工商管理学院 刘大明Chart40.450.33611111110.250.18409090910.13333333330.09423076920.06428571430.04166666670.0250.01323529410.00555555560.001315789500.00119047620.00454545450.00978260870.0
27、1666666670.0250.03461538460.04537037040.05714285710.06982758620.08333333330.09758064520.11250.1280303030.14411764710.16071428570.17777777780.19527027030.21315789470.2314102564Sheet1麦克莱恩: Operaatios Management 3/eChapter 8 库存控制库存周转率（turnover）：年销售成本/平均库存金额丰田汽车公司：22次通用汽车公司：9次库存类型周期库存（cycle inventory）：由
28、批量生产或批量订购引起。安全库存（safty inventory）：由供应和需求的不确定性引起。（缓冲库存（buffer inventory）管道库存（pipeline inventory）：生产过程中的在制品和运输途中的物品。解耦库存（decoupling inventory)：为使分销网络中的各种设施能够分散、独立运作而设立的库存。预期库存（anticipation inventory）：为应付预期会发生的供应短缺或需求增加等情形而有意储备的物品。（包括季节性库存（seasonal inventory）每类库存都有特定的目的，不是所有的库存都“坏”。库存的作用反过来会助长人们对库存的滥用用
29、以“应付”问题，而不是去解决问题，从而掩盖问题（不良的质量、供应、设备、人员等。存货单元（stock-keeping units, SKUs)库存状态（inventory level)：现有库存 + 在途库存（已订购但还未收到的库存） - 已承诺的定购量。盘点定期盘点（cycle counting）库存控制策略定期检查（periodic review）连续检查（continuous review）有效控制库存的条件需求信息提前期（lead time）成本保管成本（inventory holding cost)：资金费用（机会成本或投资报酬率），储存成本（仓库运作和折旧或租赁成本、税费、保险费、
30、过时损失、损坏损失、失窃损失等)，特殊费用（冷藏等）。通常占购买价格的10%到40%，主要是机会成本。订购/转换成本（ording or setup cost）：每次订购的交易费用或每次生产的准备费用。与定购量或生产量无关。订购和验收工作、调整设备和更换工具工作的费用。缺货成本（shortage cost）：拖欠订单（backordered）的代价或损失销售（lost sale)的机会成本。主要取决于主观估计或推测。两种确定方法：假定与缺货期的长短成比例；假定与缺货期的长短无关。两种方法的结果是不同的。EOQ哈里斯（Harris）1913年提出，经久不衰的原因：对一些主要成本的平衡能力；对不同
31、情况表现出的灵活性和适应性。在成本的粗略估计中使用EOQ（不保留安全库存）例：波敦克医院物品目前定购量Q年使用量D单价P年占用资金D$保管成本率FI15%20%30%A5001,00030.0030,000年单位保管成本CI4.5006.0009.000B303605.001,8000.7501.0001.500C7243250.0021,6007.50010.00015.000D10,0005,0000.422,1000.0630.0840.126E6001,8003.005,4000.4500.6000.900合计60,900订购成本CT13.51020%和3.5下的Q20%和3.5下的Q
32、*物品QPQ/2n=D/QQ*PQ*/2n*=D/Q*A5007500.002.0034512.3529.28B3075.0012.0050125.507.17C721800.006.0017434.7424.84D10,0002100.000.50645135.557.75E600900.003.00145217.3712.42合计12375.0023.501425.5181.46成本2475.0082.25285.10285.10TVC2557.25570.21比现在节约：订购次数增加：1987.0477.70%57.96246.63%15%和10下的Q15%和10下的Q*物品QPQ/2n
33、=D/QQ*PQ*/2n*=D/Q*A5007500.002.00671000.0015.00B3075.0012.0098244.953.67C721800.006.0034848.5312.73D10,0002100.000.501260264.583.97E600900.003.00283424.266.36合计12375.0023.502782.3241.73成本1856.25235.00417.35417.35TVC2091.25834.69在成本估计有最大误差情形下，也能节约：1256.5660.09%在不增加订购次数的情形下，能否降低成本？（定购量Q+=Q*k）20%和3.5下的
34、Q+k=n*/n =3.47物品Q+PQ+/2n+=D/Q+A1181775.958.45B174435.022.07C601506.947.17D2,237469.872.23E502753.473.58合计4941.2523.50成本988.2582.25TVC1070.50在不增加订购次数的情形下，降低成本：1486.7558.14%定购量在Q*附近调整成自然定购量，TVC增加多少？20%和3.5下的Q-r =20.00%物品Q-PQ-/2n+=D/Q-A30450.0033.33B50125.007.20C20500.0021.60D600126.008.33E150225.0012.
35、00合计1426.0082.47成本285.20288.63TVC573.83定购量在Q*的基础上调整成自然定购量，TVC增加:3.630.64%稳健性（robustness）定购量在EOQ附近的小偏差只会引起TVC根小幅度的增加。定购量过小引起的成本增加要比定购量过大引起的成本增加严重得多。对于物品A，20%和3.5的QQQTVCTVC200%102341.5766.67%175%94319.0555.68%150%85297.1645.00%125%77276.1034.72%100%68256.1725.00%75%60237.8816.07%50%51222.028.33%25%432
36、10.062.50%034204.940.00%-25%26213.484.17%-50%17256.1725.00%-75%9435.50112.50%年库存金额（annual dollar volume）D$，是影响成本的关键：D$越高，n越大，TVC越大。因此，对应的物品就是A类物品。数量折扣（discount）或整体折扣（all-units discount）对于物品A，20%和3.5的QQPCI=FIPQ*TVC*Q*TVC*0205010.0026.4650264.582040408.0029.5840236.6440306.0034.1630204.9440.0030207.50
37、QTVC1550308.331650298.751750290.881850284.441950279.212040255.002140250.672240247.092340244.172440241.832540240.002640238.622740237.632840237.002940236.693040236.673140236.903240237.383340238.063440238.943540240.003640241.223740242.593840244.113940245.744030207.504130208.374230209.334330210.404430211