Quản trị kinh doanh - Chapter 13: Inventory management

Chapter 13Inventory ManagementLecture OutlineElements of Inventory ManagementInventory Control SystemsEconomic Order Quantity ModelsQuantity DiscountsReorder PointOrder Quantity for a Periodic Inventory SystemCopyright 2011 John Wiley & Sons, Inc.13-2What Is Inventory?Stock of items kept to meet future demandPurpose of inventory managementhow many units to orderwhen to orderCopyright 2011 John Wiley & Sons, Inc.13-3Supply Chain ManagementBullwhip effectdemand information is distorted as it moves away from the end-use customerhigher safety stock inventories to are stored to compensateSeasonal or cyclical demandInventory provides independence from vendorsTake advantage of price discountsInventory provides independence between stages and avoids work stoppagesCopyright 2011 John Wiley & Sons, Inc.13-4Quality Management in the Supply ChainCustomers usually perceive quality service as availability of goods they want when they want themInventory must be sufficient to provide high-quality customer service in QMCopyright 2011 John Wiley & Sons, Inc.13-5Types of InventoryRaw materialsPurchased parts and suppliesWork-in-process (partially completed) products (WIP)Items being transportedTools and equipmentCopyright 2011 John Wiley & Sons, Inc.13-6Two Forms of DemandCopyright 2011 John Wiley & Sons, Inc.13-7DependentDemand for items used to produce final products Tires for autos are a dependent demand itemIndependentDemand for items used by external customersCars, appliances, computers, and houses are examples of independent demand inventoryInventory CostsCarrying costcost of holding an item in inventoryOrdering costcost of replenishing inventoryShortage costtemporary or permanent loss of sales when demand cannot be metCopyright 2011 John Wiley & Sons, Inc.13-8Inventory Control SystemsCopyright 2011 John Wiley & Sons, Inc.13-9Continuous system (fixed-order-quantity)constant amount ordered when inventory declines to predetermined levelPeriodic system (fixed-time-period)order placed for variable amount after fixed passage of timeABC ClassificationClass A5 – 15 % of units70 – 80 % of value Class B30 % of units15 % of valueClass C50 – 60 % of units 5 – 10 % of valueCopyright 2011 John Wiley & Sons, Inc.13-10ABC ClassificationCopyright 2011 John Wiley & Sons, Inc.13-11 1 $ 60 90 2 350 40 3 30 130 4 80 60 5 30 100 6 20 180 7 10 170 8 320 50 9 510 60 10 20 120 PART UNIT COST ANNUAL USAGEABC ClassificationCopyright 2011 John Wiley & Sons, Inc.13-12Example 10.1 9 $30,600 35.9 6.0 6.0 8 16,000 18.7 5.0 11.0 2 14,000 16.4 4.0 15.0 1 5,400 6.3 9.0 24.0 4 4,800 5.6 6.0 30.0 3 3,900 4.6 10.0 40.0 6 3,600 4.2 18.0 58.0 5 3,000 3.5 13.0 71.0 10 2,400 2.8 12.0 83.0 7 1,700 2.0 17.0 100.0 TOTAL % OF TOTAL % OF TOTAL PART VALUE VALUE QUANTITY % CUMMULATIVEABC$85,400ABC ClassificationCopyright 2011 John Wiley & Sons, Inc.13-13Example 10.1 % OF TOTAL % OF TOTAL CLASS ITEMS VALUE QUANTITY A 9, 8, 2 71.0 15.0 B 1, 4, 3 16.5 25.0 C 6, 5, 10, 7 12.5 60.0Economic Order Quantity (EOQ) ModelsEOQoptimal order quantity that will minimize total inventory costsBasic EOQ modelProduction quantity modelCopyright 2011 John Wiley & Sons, Inc.13-14Assumptions of Basic EOQ ModelDemand is known with certainty and is constant over timeNo shortages are allowedLead time for the receipt of orders is constantOrder quantity is received all at onceCopyright 2011 John Wiley & Sons, Inc.13-15Inventory Order CycleCopyright 2011 John Wiley & Sons, Inc.13-16Demand rateTimeLead timeLead timeOrder placedOrder placedOrder receiptOrder receiptInventory LevelReorder point, ROrder quantity, Q0Average inventory Q2EOQ Cost ModelCopyright 2011 John Wiley & Sons, Inc.13-17Co - cost of placing order D - annual demandCc - annual per-unit carrying cost Q - order quantityAnnual ordering cost =CoDQAnnual carrying cost =CcQ2Total cost = +CoDQCcQ2EOQ Cost ModelCopyright 2011 John Wiley & Sons, Inc.13-18TC = +CoDQCcQ2= – +CoDQ2Cc2TCQ0 = – +C0DQ2Cc2Qopt =2CoDCcDeriving QoptProving equality of costs at optimal point=CoDQCcQ2Q2 =2CoDCcQopt =2CoDCcEOQ Cost ModelCopyright 2011 John Wiley & Sons, Inc.13-19Order Quantity, QAnnual cost ($)Total CostCarrying Cost =CcQ2Slope = 0Minimum total costOptimal order QoptOrdering Cost =CoDQEOQ ExampleCopyright 2011 John Wiley & Sons, Inc.13-20Cc = $0.75 per gallon Co = $150 D = 10,000 gallonsQopt =2CoDCcQopt =2(150)(10,000)(0.75)Qopt = 2,000 gallonsTCmin = +CoDQCcQ2TCmin = +(150)(10,000)2,000(0.75)(2,000)2TCmin = $750 + $750 = $1,500 Orders per year = D/Qopt = 10,000/2,000 = 5 orders/year Order cycle time = 311 days/(D/Qopt) = 311/5 = 62.2 store daysProduction Quantity ModelOrder is received gradually, as inventory is simultaneously being depletedAKA non-instantaneous receipt modelassumption that Q is received all at once is relaxedp - daily rate at which an order is received over time, a.k.a. production rated - daily rate at which inventory is demandedCopyright 2011 John Wiley & Sons, Inc.13-21Production Quantity ModelCopyright 2011 John Wiley & Sons, Inc.13-22Q(1-d/p)Inventorylevel(1-d/p)Q2Time0Orderreceipt periodBeginorderreceiptEndorderreceiptMaximuminventory levelAverageinventory levelProduction Quantity ModelCopyright 2011 John Wiley & Sons, Inc.13-23p = production rate d = demand rate Maximum inventory level = Q - d = Q 1 -QpdpAverage inventory level = 1 -Q2dpTC = + 1 -dpCoDQCcQ2Qopt =2CoDCc 1 - dpProduction Quantity ModelCopyright 2011 John Wiley & Sons, Inc.13-24Cc = $0.75 per gallon Co = $150 D = 10,000 gallonsd = 10,000/311 = 32.2 gallons per day p = 150 gallons per dayQopt = = = 2,256.8 gallons2CoDCc 1 - dp2(150)(10,000)0.75 1 - 32.2150TC = + 1 - = $1,329dpCoDQCcQ2Production run = = = 15.05 days per orderQp2,256.8150Production Quantity ModelCopyright 2011 John Wiley & Sons, Inc.13-25Number of production runs = = = 4.43 runs/yearDQ10,0002,256.8 Maximum inventory level = Q 1 - = 2,256.8 1 - = 1,772 gallonsdp32.2150Solution of EOQ Models With ExcelCopyright 2011 John Wiley & Sons, Inc.13-26The optimal ordersize, Q, in cell D8Solution of EOQ Models With ExcelCopyright 2011 John Wiley & Sons, Inc.13-27The formula for Qin cell D10=(D4*D5/D10)+(D3*D10/2)*(1-(D7/D8))=D10*(1-(D7/D8))Solution of EOQ Models With OM ToolsCopyright 2011 John Wiley & Sons, Inc.13-28Quantity DiscountsCopyright 2011 John Wiley & Sons, Inc.13-29Price per unit decreases as order quantity increasesTC = + + PDCoDQCcQ2whereP = per unit price of the itemD = annual demandQuantity Discount ModelCopyright 2011 John Wiley & Sons, Inc.13-30QoptCarrying cost Ordering cost Inventory cost ($)Q(d1 ) = 100Q(d2 ) = 200TC (d2 = $6 ) TC (d1 = $8 ) TC = ($10 ) ORDER SIZE PRICE0 - 99 $10100 – 199 8 (d1)200+ 6 (d2)Quantity DiscountCopyright 2011 John Wiley & Sons, Inc.13-31 QUANTITY PRICE 1 - 49 $1,400 50 - 89 1,100 90+ 900 Co = $2,500 Cc = $190 per TV D = 200 TVs per yearQopt = = = 72.5 TVs2CoDCc2(2500)(200)190TC = + + PD = $233,784 CoDQoptCcQopt2For Q = 72.5TC = + + PD = $194,105CoDQCcQ2For Q = 90Quantity Discount Model With ExcelCopyright 2011 John Wiley & Sons, Inc.13-32=(D4*D5/E10)+(D3*E10/2)+C10*D5=IF(D10>B10,D10,B10)Reorder PointCopyright 2011 John Wiley & Sons, Inc.13-33Inventory level at which a new order is placed R = dLwhere d = demand rate per period L = lead timeReorder PointCopyright 2011 John Wiley & Sons, Inc.13-34Demand = 10,000 gallons/yearStore open 311 days/yearDaily demand = 10,000 / 311 = 32.154 gallons/dayLead time = L = 10 daysR = dL = (32.154)(10) = 321.54 gallonsSafety Stock Safety stockbuffer added to on hand inventory during lead timeStockout an inventory shortageService level probability that the inventory available during lead time will meet demandP(Demand during lead time <= Reorder Point)Copyright 2011 John Wiley & Sons, Inc.13-35Variable Demand With Reorder PointCopyright 2011 John Wiley & Sons, Inc.13-36Reorderpoint, RQLTTimeLTInventory level0Reorder Point With Safety StockCopyright 2011 John Wiley & Sons, Inc.13-37Reorderpoint, RQLTTimeLTInventory level0Safety StockReorder Point With Variable DemandCopyright 2011 John Wiley & Sons, Inc.13-38R = dL + zd Lwhere d = average daily demand L = lead time d = the standard deviation of daily demand z = number of standard deviations corresponding to the service level probability zd L = safety stockReorder Point For a Service LevelCopyright 2011 John Wiley & Sons, Inc.13-39Probability of meeting demand during lead time = service levelProbability of a stockoutRSafety stockdLDemandzd LReorder Point For Variable DemandCopyright 2011 John Wiley & Sons, Inc.13-40The paint store wants a reorder point with a 95% service level and a 5% stockout probability d = 30 gallons per day L = 10 days d = 5 gallons per dayFor a 95% service level, z = 1.65 R = dL + z d L = 30(10) + (1.65)(5)( 10) = 326.1 gallons Safety stock = z d L = (1.65)(5)( 10) = 26.1 gallonsDetermining Reorder Point with ExcelCopyright 2011 John Wiley & Sons, Inc.13-41The reorder pointformula in cell E7Order Quantity for a Periodic Inventory SystemCopyright 2011 John Wiley & Sons, Inc.13-42Q = d(tb + L) + zd tb + L - Iwhere d = average demand rate tb = the fixed time between orders L = lead time sd = standard deviation of demand zd tb + L = safety stock I = inventory levelPeriodic Inventory SystemCopyright 2011 John Wiley & Sons, Inc.13-43Fixed-Period Model With Variable DemandCopyright 2011 John Wiley & Sons, Inc.13-44 d = 6 packages per day sd = 1.2 packages tb = 60 days L = 5 days I = 8 packages z = 1.65 (for a 95% service level) Q = d(tb + L) + zd tb + L - I = (6)(60 + 5) + (1.65)(1.2) 60 + 5 - 8 = 397.96 packagesFixed-Period Model with ExcelCopyright 2011 John Wiley & Sons, Inc.13-45Formula for ordersize, Q, in cell D10Copyright 2011 John Wiley & Sons, Inc.13-46Copyright 2011 John Wiley & Sons, Inc.All rights reserved. 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