全球化趋势下一般企业经常面对问题

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1、2-1Inventory Management全球化趨勢下一般企業經常面對的問題全球化趨勢下一般企業經常面對的問題l高度需求變動高度需求變動l訂貨前置時間長訂貨前置時間長l不可靠的供應程序不可靠的供應程序l大量的儲存單位大量的儲存單位(SKUs)2-2Inventory Management案例案例l三工業的問題三工業的問題(前置時間前置時間)l手機的產品壽命週期：手機的產品壽命週期：200000元元 (產品壽產品壽週期週期需求變異需求變異)lIpad 對電子書的衝擊對電子書的衝擊 (競爭競爭需求變異需求變異)l新機推出後一個月新機推出後一個月IPhone跌跌2千；千；hTC跌跌5千；三星跌千

2、；三星跌3千千l智慧型手機可能帶來衝擊智慧型手機可能帶來衝擊l電子書電子書l遊戲機遊戲機l隨身聽隨身聽l衛星導航衛星導航2-3Inventory ManagementWhy Is Inventory Important?1Distribution and inventory (logistics) costs are quite substantial Total U.S. Manufacturing Inventories ($m):l1992-01-31: $m 808,773 l1996-08-31: $m 1,000,774 l2006-05-31: $m 1,324,108 Inve

3、ntory-Sales Ratio (U.S. Manufacturers):l1992-01-01: 1.56 l2006-05-01: 1.25 2-4Inventory ManagementlGMs production and distribution networkl20,000 supplier plantsl133 parts plantsl31 assembly plantsl11,000 dealerslFreight transportation costs: $4.1 billion (60% for material shipments) lGM inventory v

4、alued at $7.4 billion (70%WIP; Rest Finished Vehicles)lDecision tool to reduce:lcombined corporate cost of inventory and transportation. l26% annual cost reduction by adjusting: lShipment sizes (inventory policy) lRoutes (transportation strategy)Why Is Inventory Important?22-5Inventory ManagementInv

5、entorylWhere do we hold inventory?lSuppliers and manufacturerslwarehouses and distribution centerslretailerslTypes of InventorylWIP (work in process)lraw materialslfinished goods2-6Inventory ManagementThe reasons of holding inventorylUnexpected changes in customer demandlThe short life cycle of an i

6、ncreasing number of products.lThe presence of many competing products in the marketplace.lUncertainty in the quantity and quality of the supply, supplier costs and delivery times.lDelivery Lead Time, Capacity limitationslEconomies of scale (transportation cost)2-7Inventory Management問題討論問題討論l小米小米3上市

7、上市，對智慧型手機市場的衝擊對智慧型手機市場的衝擊l3DS上市對掌上型遊戲機市場的衝擊上市對掌上型遊戲機市場的衝擊2-8Inventory ManagementGoals: Reduce Cost, Improve Service Example1lBy effectively managing inventory:lWal-Mart became the largest retail company utilizing efficient inventory managementlGM has reduced parts inventory and transportation costs

8、by 26% annually2-9Inventory ManagementGoals: Reduce Cost, Improve Service Example2lBy not managing inventory successfullylIn 1994, “IBM continues to struggle with shortages in their ThinkPad line” (WSJ, Oct 7, 1994)lIn 1993, “Dell Computers predicts a loss; Stock plunges. Dell acknowledged that the

9、company was sharply off in its forecast of demand, resulting in inventory write downs” (WSJ, August 1993)2-10Inventory ManagementInventory Management vs. Demand ForecastslUncertain demand makes demand forecast critical for inventory related decisions:lWhat to order?lWhen to order?lHow much is the op

10、timal order quantity?lApproach includes a set of techniqueslINVENTORY POLICY!2-11Inventory ManagementSupply Chain Factors in Inventory Policy1lEstimation of customer demandlReplenishment lead timelThe number of different products being consideredlThe length of the planning horizonlService level requ

11、irements2-12Inventory ManagementSupply Chain Factors in Inventory Policy2lCostslOrder cost (or setup cost): lProduct cost lTransportation costlInventory holding cost (or inventory carrying cost):lState taxes, property taxes, and insurance on inventorieslMaintenance costslObsolescence costlOpportunit

12、y costs2-13Inventory Management2.2 Single Stage Inventory ControllSingle supply chain stagelVariety of techniqueslEconomic Lot Size ModellDemand UncertaintylSingle Period ModelslInitial InventorylMultiple Order OpportunitieslContinuous Review PolicylVariable Lead TimeslPeriodic Review PolicylService

13、 Level Optimization2-14Inventory ManagementExamplelBook Store Mug SaleslDemand is constant, at 20 units a weeklFixed order cost of $12.00, no lead timelHolding cost of 25% of inventory value annuallylMugs cost $1.00, sell for $5.00lQuestionlHow many, when to order?2-15Inventory Management2.2.1 Econo

14、mic Lot Size Model (EOQ)(Ford W. Harris, 1915)TimeInventoryOrderSizeNote: No Stockouts Order when no inventory Order Size determines policyAvg. InvenCycle Time =T2-16Inventory ManagementAssumptionslD items per day: Constant demand ratelQ items per order: Order quantities are fixed, i.e., each time t

15、he warehouse places an order, it is for Q items.lK, fixed setup cost, incurred every time the warehouse places an order.lh, inventory carrying cost accrued per unit held in inventory per day that the unit is held (also known as, holding cost)lLead time = 0 (the time that elapses between the placemen

16、t of an order and its receipt)lInitial inventory = 0lPlanning horizon is long (infinite).2-17Inventory ManagementDeriving EOQlTotal cost at every cycle:lCycle time T =Q/D lAverage total cost per unit time: h T QK22hQQKDhKDQ2*2-18Inventory ManagementEOQ:Total CostTotal CostOrder CostHolding Cost2-19I

17、nventory ManagementEOQ: Optimal Order QuantitylSo for our problem (Mug sales), the optimal quantity is 3162KDQh*2 20 52 12Q3160.25 12-20Inventory ManagementEOQ: Important ObservationslTrade-off between set-up costs and holding costs when determining order quantity. In fact, we order so that these co

18、sts are equal per unit timelTotal Cost is not particularly sensitive to the optimal order quantity2-21Inventory ManagementSensitivity Analysisb.5.8.911.11.21.52Increase in cost25%2.5%0.5% 0.4%1.6% 8.9%25%Total inventory cost relatively insensitive to order quantitiesActual order quantity: Q Q is a m

19、ultiple b of the optimal order quantity Q*. For a given b, the quantity ordered is Q = bQ* 2-22Inventory Management2.2.2 The Effect of Demand UncertaintylMost companies treat the world as if it were predictable:lProduction and inventory planning are based on forecasts of demand made far in advance o

20、f the selling seasonlCompanies are aware of demand uncertainty when they create a forecast, but they design their planning process as if the forecast truly represents realitylRecent technological advances have increased the level of demand uncertainty:lShort product life cycles lIncreasing product v

21、ariety2-23Inventory Management問題問題liPhone 5S 與與 iPhone 5C的銷售量的銷售量(參考參考iPhone 4？)lNew hTC one 的銷售量的銷售量(參考蝴蝶機參考蝴蝶機？)2-24Inventory ManagementThree principles of all forecasting techniqueslThe forecast is always wronglIt is difficult to match supply and demandlThe longer the forecast horizon, the worse

22、the forecastlIt is even more difficult if one needs to predict customer demand for a long period of timelAggregate forecasts are more accurate.lMore difficult to predict customer demand for individual SKUslMuch easier to predict demand across all SKUs within one product family2-25Inventory Managemen

23、t2.2.3. Single Period ModelsShort lifecycle products(例如，例如，ipad)lOne ordering opportunity onlylOrder quantity to be decided before demand occurslOrder Quantity Demand = Dispose excess inventorylOrder Quantity Lose sales/profits2-26Inventory ManagementSingle Period ModelslUsing historical datalidenti

24、fy a variety of demand scenarios ldetermine probability each of these scenarios will occurlGiven a specific inventory policyldetermine the profit associated with a particular scenariolgiven a specific order quantity lweight each scenarios profit by the likelihood that it will occurldetermine the ave

25、rage, or expected profit for a particular ordering quantity. lOrder the quantity that maximizes the average profit.2-27Inventory ManagementSwimsuit productionlFashion items have short life cycles, high variety of competitors (智慧型手機？智慧型手機？)lSwimsuit productionlNew designs are completedlOne production

26、 opportunitylBased on past sales, knowledge of the industry, and economic conditions, the marketing department has a probabilistic forecastlThe forecast averages about 13,000, but there is a chance that demand will be greater or less than this.Example Swimsuit production2-28Inventory ManagementDeman

27、d ScenariosDemand Scenarios0%5%10%15%20%25%30%80001000012000140001600018000Sales Probability Example Swimsuit production11%28%11%22%18%10%2-29Inventory ManagementCostslProduction cost per unit (C): $80lSelling price per unit (S): $125lSalvage value per unit (V): $20lFixed production cost (F): $100,0

28、00lQ is production quantityExample Swimsuit production2-30Inventory ManagementTwo ScenarioslScenario One:lSuppose you make 10,000 swimsuits and demand ends up being 12,000 swimsuits.lProfit = 125(10,000) - 80(10,000) - 100,000 = $350,000lScenario Two:lSuppose you make 10,000 swimsuits and demand end

29、s up being 8,000 swimsuits.lProfit = 125(8,000) - 80(10,000) - 100,000 + 20(2,000) = $ 140,000Example Swimsuit production2-31Inventory ManagementProbability of Profitability Scenarios with Production = 10,000 UnitslProbability of demand being 8000 units = 11%lProbability of profit of $140,000 = 11%l

30、Probability of demand being 12000 units = 28%lProbability of profit of $350,000 = 28%lTotal profit = Weighted average of profit scenarios 2-32Inventory ManagementExpected profit of production quantity Q1lDi=the ith demandlPi=the profit of production quantity Q at demand Diiiiii125 D80Q 10000020 QD i

31、f QDP125 Q80Q 100000 if QD 2-33Inventory ManagementExpected profit of production quantity Q2lf(Pi)=the probability of profit P at demand Di When production quantity=QExpect Profit of Q: E(P) iiiE PPf P2-34Inventory ManagementSwimsuit production SolutionlFind order quantity that maximizes weighted av

32、erage profit.lQuestion: Will this quantity be less than, equal to, or greater than average demand?Example Swimsuit production2-35Inventory ManagementOrder Quantity that Maximizes Expected ProfitFIGURE 2-6: Average profit as a function of production quantity2-36Inventory ManagementRelationship Betwee

33、n Optimal Quantity and Average DemandlCompare marginal profit of selling an additional unit and marginal cost of not selling an additional unitlMarginal profit/unit = Selling Price - Variable Ordering (or, Production) Cost lMarginal cost/unit =Variable Ordering (or, Production) Cost - Salvage Value

34、lIf Marginal Profit Marginal Cost = Optimal Quantity Average DemandlIf Marginal Profit Optimal Quantity Marginal Profit= optimal production quantity profit achieved if we produce to increase inventory to 12,000 units lIf we produce, the most we can make on average is a profit of $371,000. lSame aver

35、age profit with initial inventory of 8,245 units and not producing anything. lIf initial inventory produce to raise the inventory level to 12,000 units. lIf initial inventory is at least 8,245 units, we should not produce anything (s, S) policy or (min, max) policy Manufacturer Initial Inventory = 1

36、0,000 Example Swimsuit production2-48Inventory Management(s, S) PolicieslFor some starting inventory levels, it is better to not start productionlIf we start, we always produce to the same levellThus, we use an (s,S) policy. If the inventory level is below s, we produce up to S. ls is the reorder po

37、int, and S is the order-up-to levellThe difference between the two levels is driven by the fixed costs associated with ordering, transportation, or manufacturing2-49Inventory Management2.2.5. Multiple Order Opportunities1REASONSlTo balance annual inventory holding costs and annual fixed order costs.

38、 lTo satisfy demand occurring during lead time. lTo protect against uncertainty in demand.2-50Inventory Management2.2.5. Multiple Order Opportunities2lTWO POLICIESlContinuous review policy(持續檢視政策持續檢視政策)linventory is reviewed continuouslylan order is placed when the inventory reaches a particular lev

39、el or reorder point. linventory can be continuously reviewed (computerized inventory systems are used)lPeriodic review policy(週期檢視政策週期檢視政策)linventory is reviewed at regular intervals lappropriate quantity is ordered after each review. lit is impossible or inconvenient to frequently review inventory

40、and place orders if necessary.2-51Inventory Management2.2.6. Continuous Review Policy AssumptionslDaily demand is random and follows a normal distribution. lEvery time the distributor places an order from the manufacturer, the distributor pays a fixed cost, K, plus an amount proportional to the quan

41、tity ordered.lInventory holding cost is charged per item per unit time.lInventory level is continuously reviewed, and if an order is placed, the order arrives after the appropriate lead time.lIf a customer order arrives when there is no inventory on hand to fill the order (i.e., when the distributor

42、 is stocked out), the order is lost.(不考慮缺貨不考慮缺貨)lThe distributor specifies a required service level. 2-52Inventory ManagementThe (Q,R) Policyl(Q,R ) Policy: Whenever the inventory position drops below a certain level, R, we order to raise the inventory position to level Q.lThe reorder point (R) is a

43、 function of:lThe Lead TimelAverage demandlDemand variabilitylService level2-53Inventory Management NotationlAVG = average daily demandlSTD = standard deviation of daily demandlL = replenishment lead time in dayslh = holding cost of one unit for one daylK = fixed cost (setup cost)l = service level.

44、This implies that the probability of stocking out is 1 - lAlso, the Inventory Position at any time is the actual inventory plus items already ordered, but not yet delivered.2-54Inventory Management Analysis1lThe reorder point - (R) has two components:l1. To account for average demand during lead tim

45、e:l2. To account for deviations from average (we call this safety stock) where z is chosen from statistical tables to ensure that the probability of stock-outs during leadtime is 100%-SL.LAVGzSTDL2-55Inventory Management Analysis2lreorder point - (R):lThe total order-up-to level is (S)(倉庫容倉庫容量量):lTh

46、e average inventory level is:LAVGZSTDL2KAVGQhSQR2QZSTDL2-56Inventory ManagementService Level & Safety Factor, zService Level (1-)90%91%92%93%94% 95%96%97%98%99%99.9%z1.291.341.411.481.561.651.751.882.052.333.08z is chosen from statistical tables to ensure that the probability of stock-outs during le

47、ad time is exactly 1 - 2-57Inventory ManagementInventory Level Over TimeLSTDzInventory level before receiving an order =Inventory level after receiving an order =Average Inventory =LSTDzQLSTDzQ2FIGURE 2-9: Inventory level as a function of time in a (Q,R) policy2-58Inventory ManagementContinuous Revi

48、ew Policy Example1lA distributor of TV sets that orders from a manufacturer and sells to retailerslFixed ordering cost = $4,500lCost of a TV set to the distributor = $250lAnnual inventory holding cost = 18% of product costlReplenishment lead time = 2 weekslExpected service level = 97%(z=1.9)2-59Inve

49、ntory ManagementMonthSeptOct Nov. Dec. Jan. Feb. Mar. Apr. May June July AugSales 200 152 100 221 287 176 151 198 246 309 98 156Continuous Review Policy Example2Average monthly demand = 191.17 Standard deviation of monthly demand = 66.53Average weekly demand = Average Monthly Demand/4.3 =44.58Standa

50、rd deviation of weekly demand = Monthly standard deviation/4.3 =32.082-60Inventory ManagementParameterAverage weekly demandStandard deviation of weekly demandAverage demand during lead timeSafety stockReorder pointValue44.5832.0889.1686.2017687. 05225018. 0Weekly holding cost = Optimal order quantit

51、y = 67987.58.44500, 42QAverage inventory level = 679/2 + 86.20 = 426Continuous Review Policy Example32-61Inventory Management2.2.7.變動的前置時間與需求變動的前置時間與需求1l在許多情況下，運送至倉庫的運輸前置時間被假在許多情況下，運送至倉庫的運輸前置時間被假設是固定的，而且是預先知道，實則不然。在許設是固定的，而且是預先知道，實則不然。在許多實際情況下，多實際情況下，運送至倉庫的前置時間，必須假運送至倉庫的前置時間，必須假設為常態機率分配，平均前置時間以設為常態機

52、率分配，平均前置時間以AVGL表示表示及標準差以及標準差以STDL表示表示。在此情況下，再訂購點。在此情況下，再訂購點R的計算如下：的計算如下：222 RAVGAVGLzAVGLSTDAVGSTDL2-62Inventory Management2.2.7.變動的前置時間與需求變動的前置時間與需求2l其中其中AVG AVGL表示平均前置時間內的平均需表示平均前置時間內的平均需求，而求，而 l為平均前置時間內的平均需求標準差。為平均前置時間內的平均需求標準差。l因此，應維持之安全存貨為：因此，應維持之安全存貨為：222zAVGLSTDAVGSTDL222AVGL STDAVGSTDL2-63In

53、ventory Management2.2.7.變動的前置時間與需求變動的前置時間與需求32KA V GQhOrder Quantity =l如前述，訂購量上限為安全存貨加上如前述，訂購量上限為安全存貨加上Q和前和前置時間內平均需求的最大值，也就是：置時間內平均需求的最大值，也就是：222SQ AVG AVGL z AVGL STDAVGSTDL2-64Inventory ManagementlInventory level is reviewed periodically at regular intervals lAn appropriate quantity is ordered aft

54、er each review lTwo Cases:lShort Intervals (e.g. Daily)lDefine two inventory levels s and S lDuring each inventory review, if the inventory position falls below s, order enough to raise the inventory position to S. l(s, S) policy2.2.8. Periodic Review Policy12-65Inventory Management(s,S) policylCalc

55、ulate the Q and R values as if this were a continuous review modellSet s equal to RlSet S equal to R+Q.2-66Inventory ManagementA View of (s, S) PolicyTimeInventory LevelSs0LeadTimeLeadTimeInventory Position2-67Inventory ManagementlTwo Cases:lLonger Intervals (e.g. Weekly or Monthly)lMay make sense t

56、o always order after an inventory level review. lDetermine a target inventory level, the base-stock levellDuring each review period, the inventory position is reviewed lOrder enough to raise the inventory position to the base-stock level. lBase-stock level policy2.2.8. Periodic Review Policy22-68Inv

57、entory ManagementBase-Stock Level Policy1 lDetermine a target inventory level, the base-stock levellEach review period, review the inventory position is reviewed and order enough to raise the inventory position to the base-stock levellAssume:r = length of the review periodL = lead time AVG = average

58、 daily demand STD = standard deviation of this daily demand. 2-69Inventory ManagementBase-Stock Level Policy2lEach review echelon, inventory position is raised to the base-stock level.lThe base-stock level includes three components:lAverage demand during (r +L) days (the time until the next order ar

59、riveslSafety stock during that timelAmount on hand at order time (A)2-70Inventory ManagementBase-Stock Level Policy3l安全存量為安全存量為 l基本存貨水準基本存貨水準(base-stock level) l而進貨的前一刻，前置時間內的需求已耗用，只剩安全而進貨的前一刻，前置時間內的需求已耗用，只剩安全存量存量l因此平均存貨水準等於：因此平均存貨水準等於： zSTDL() LAVGzSTDLzSTDL2AVGzSTDL (訂購量訂購量=保護期間的預期需求保護期間的預期需求+安全存量

60、安全存量-再訂購點時的現有數量再訂購點時的現有數量)2-71Inventory ManagementBase-Stock Level Policy 4FIGURE 2-10: Inventory level as a function of time in a periodic review policy2-72Inventory ManagementlAssume: ldistributor places an order for TVs every 3 weekslLead time is 2 weekslBase-stock level needs to cover 5 weekslAv

61、erage demand = 44.58 x 5 = 222.9lSafety stock = lBase-stock level = 223 + 136 = 359lAverage inventory level =lDistributor keeps 5 (= 203.17/44.58) weeks of supply.Base-Stock Level Policy Example 17.203508.329 . 1258.4431.9 32.0851362-73Inventory ManagementlOptimal inventory policy assumes a specific

62、 service level target. lWhat is the appropriate level of service? lMay be determined by the downstream customerlRetailer may require the supplier, to maintain a specific service levellSupplier will use that target to manage its own inventorylFacility may have the flexibility to choose the appropriat

63、e level of service2.2.9. Service Level Optimization2-74Inventory ManagementService Level OptimizationFIGURE 2-11: Service level inventory versus inventory level as a function of lead time2-75Inventory ManagementTrade-OffslEverything else being equal:lthe higher the service level, the higher the inve

64、ntory level. lfor the same inventory level, the longer the lead time to the facility, the lower the level of service provided by the facility. lthe lower the inventory level, the higher the impact of a unit of inventory on service level and hence on expected profit2-76Inventory ManagementRetail (多樣化

65、產品多樣化產品) StrategylGiven a target service level across all products determine service level for each SKU so as to maximize expected profit. lEverything else being equal, service level will be higher for products with:lhigh profit marginlhigh volumellow variabilitylshort lead time2-77Inventory Managem

66、entProfit Optimization and Service Level1FIGURE 2-12: Service level optimization by SKU2-78Inventory ManagementlTarget inventory level = 95% across all products. lService level 99% for many products with high profit margin, high volume and low variability. lService level 95% for products with low profit margin, low volume and high variability. Profit Optimization and Service Level22-79Inventory ManagementRisk Pooling Question1lConsider these two systems:Market TwoSupplierWarehouse OneWarehouse T