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12 – 1 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Inventory Management 12 For Operations Management, 9e by Krajewski/Ritzman/Malhotra © 2010 Pearson Education PowerPoint Slides by Jeff Heyl

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12 – 2 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Inventory Management Inventories are important to all types of organizations Inventory must be managed…… They have to be counted, paid for, used in operations, used to satisfy customers, and managed Too much inventory reduces profitability Too little inventory damages customer confidence Inventory trade-offs

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12 – 3 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. ABC Analysis ABC Analysis is one tool for managing: Stock-keeping units (SKU) Identify the classes so management can control inventory levels A Pareto chart Cycle counting

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12 – 4 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. ABC Analysis Insert inventory chart here

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12 – 5 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall Percentage of SKUs Percentage of dollar value Class C Class A Class B ABC Analysis Figure 12.1 – Typical Chart Using ABC Analysis

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12 – 6 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Economic Order Quantity The lot size, Q, that minimizes total annual inventory holding and ordering costs Five assumptions 1.Demand rate is constant and known with certainty 2.No constraints are placed on the size of each lot 3.The only two relevant costs are the inventory holding cost and the fixed cost per lot for ordering or setup 4.Decisions for one item can be made independently of decisions for other items 5.The lead time is constant and known with certainty

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12 – 7 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Economic Order Quantity Dont use the EOQ Make-to-order strategy Order size is constrained Modify the EOQ Quantity discounts Replenishment not instantaneous Use the EOQ Make-to-stock Carrying and setup costs are known and relatively stable

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12 – 8 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Calculating EOQ Inventory depletion (demand rate) Receive order 1 cycle On-hand inventory (units) Time Q Average cycle inventory Q2Q2 Figure 12.2 – Cycle-Inventory Levels

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12 – 9 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Calculating EOQ Annual holding cost (H) Annual holding cost =(Average cycle inventory) (Unit holding cost) Annual ordering cost (S) Annual ordering cost =(Number of orders/Year) (Ordering or setup costs) Total annual cycle-inventory cost (TC) Total costs =Annual holding cost + Annual ordering or setup cost HC = Q D (S) OC = 2 Q (H) TC = OC + HC

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12 – 10 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Annual cost (dollars) Lot Size ( Q ) Holding cost Ordering cost Total cost Calculating EOQ Figure 12.3 – Graphs of Annual Holding, Ordering, and Total Costs

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12 – 11 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Calculating EOQ Total annual cycle-inventory cost where C = total annual cycle-inventory cost Q = lot size H = holding cost per unit per year D = annual demand S = ordering or setup costs per lot TC = ( H ) + ( S ) Q2Q2 DQDQ

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12 – 12 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. The Cost of a Lot-Sizing Policy EXAMPLE 12.1 A museum of natural history opened a gift shop which operates 52 weeks per year. Managing inventories has become a problem. Top-selling SKU is a bird feeder. Sales are 18 units per week, the supplier charges $60 per unit. Ordering cost is $45. Annual holding cost is 25 percent of a feeders value. Management chose a 390-unit lot size. What is the annual cycle-inventory cost of the current policy of using a 390-unit lot size? Would a lot size of 468 be better?

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12 – 13 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. The Cost of a Lot-Sizing Policy What we Know: Sell bird feeders: operate 52 weeks per year d = 18 units per week D = (18 X 52) = 936 p = $60 per unit S = $45 H = (.25 X $60) = $15 Q = 390 What we want to Know: What is the annual cycle-inventory cost of the current policy of using a 390-unit lot size? Would a lot size of 468 be better?

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12 – 14 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. The Cost of a Lot-Sizing Policy SOLUTION TC = ( H ) + ( S ) Q2Q2 DQDQ The total annual cycle-inventory cost for the alternative lot size is = ($15) + ($45) = $2,925 + $108 = $3, The total annual cycle-inventory cost for the current policy is TC = ($15) + ($45) = $3,510 + $90 = $3, TC = ( H ) + ( S ) Q2Q2 DQDQ

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12 – 15 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. The Cost of a Lot-Sizing Policy 3000 – 2000 – 1000 – 0 – |||||||| Lot Size ( Q ) Annual cost (dollars) Current Q Current cost Lowest cost Best Q (EOQ) Total cost = ( H ) + ( S ) Q2Q2 DQDQ Figure 12.4 –Total Annual Cycle-Inventory Cost Function for the Bird Feeder Ordering cost = ( S ) DQDQ Holding cost = ( H ) Q2Q2

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12 – 16 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Calculating EOQ The EOQ formula: EOQ = 2 DS H Time between orders TBO EOQ = (12 months/year) EOQ D

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12 – 17 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Finding the EOQ, Total Cost, TBO EXAMPLE 12.2 For the bird feeders in Example 12.1, calculate the EOQ and its total annual cycle-inventory cost. How frequently will orders be placed if the EOQ is used? SOLUTION Using the formulas for EOQ and annual cost, we get EOQ = = 2 DS H = or 75 units 2(936)(45) 15

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12 – 18 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Finding the EOQ, Total Cost, TBO Figure 12.5 shows that the total annual cost is much less than the $3,033 cost of the current policy of placing 390-unit orders. Figure 12.5 –Total Annual Cycle-Inventory Costs Based on EOQ Using Tutor 12.2

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12 – 19 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Finding the EOQ, Total Cost, TBO When the EOQ is used, the TBO can be expressed in various ways for the same time period. TBO EOQ = EOQ D TBO EOQ = (12 months/year) EOQ D TBO EOQ = (52 weeks/year) EOQ D TBO EOQ = (365 days/year) EOQ D = = year = (12) = 0.96 month = (52) = 4.17 weeks = (365) = days

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12 – 20 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Application 12.1 Suppose that you are reviewing the inventory policies on an $80 item stocked at a hardware store. The current policy is to replenish inventory by ordering in lots of 360 units. Additional information is: D = 60 units per week, or 3,120 units per year S = $30 per order H = 25% of selling price, or $20 per unit per year What is the EOQ? EOQ = = 2 DS H = 97 units 2(3,120)(30) 20 SOLUTION

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12 – 21 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Current PolicyEOQ Policy Application 12.1 What is the total annual cost of the current policy ( Q = 360), and how does it compare with the cost with using the EOQ? Q = 360 units Q = 97 units C = 3, C = $3,860 C = (360/2)(20) + (3,120/360)(30) C = C = $1,935 C = (97/2)(20) + (3,120/97)(30)

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12 – 22 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Application 12.1 What is the time between orders (TBO) for the current policy and the EOQ policy, expressed in weeks? TBO 360 = TBO EOQ = (52 weeks per year) = 6 weeks 360 3,120 (52 weeks per year) = 1.6 weeks 97 3,120 SOLUTION

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12 – 23 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Managerial Insights TABLE 12.1 | SENSITIVITY ANALYSIS OF THE EOQ ParameterEOQParameter Change EOQ Change Comments Demand Increase in lot size is in proportion to the square root of D. Order/Setup Costs Weeks of supply decreases and inventory turnover increases because the lot size decreases. Holding Costs Larger lots are justified when holding costs decrease. 2DSH2DSH 2DSH2DSH 2 DS H

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12 – 24 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Inventory Control Systems Two important questions about ordering: How much? When? Nature of demand Independent demand - Forecasted Dependent demand - Calculated

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12 – 25 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Inventory Control Systems Continuous review ( Q ) system Reorder point system (ROP) and fixed order quantity system For independent demand items Tracks inventory position (IP) Includes scheduled receipts (SR), on-hand inventory (OH), and back orders (BO) Inventory position =On-hand inventory + Scheduled receipts – Backorders IP = OH + SR – BO

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12 – 26 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Selecting the Reorder Point Time On-hand inventory TBO L L L Order placed Order placed Order placed IP R Q Q Q OH Order received Order received Order received Order received Figure 12.6 – Q System When Demand and Lead Time Are Constant and Certain

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12 – 27 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Application 12.2 The on-hand inventory is only 10 units, and the reorder point R is 100. There are no backorders and one open order for 200 units. Should a new order be placed? IP = OH + SR – BO = – 0 = 210 R = 100 SOLUTION Decision: Place no new order

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12 – 28 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Placing a New Order EXAMPLE 12.3 Demand for chicken soup at a supermarket is always 25 cases a day and the lead time is always 4 days. The shelves were just restocked with chicken soup, leaving an on-hand inventory of only 10 cases. No backorders currently exist, but there is one open order in the pipeline for 200 cases. What is the inventory position? Should a new order be placed? SOLUTION R =Total demand during lead time = (25)(4) = 100 cases = – 0 = 210 cases IP = OH + SR – BO

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12 – 29 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Continuous Review Systems Selecting the reorder point with variable demand and constant lead time Reorder point=Average demand during lead time + Safety stock = dL + safety stock where d = average demand per week (or day or months) L = constant lead time in weeks (or days or months)

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12 – 30 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Continuous Review Systems Time On-hand inventory TBO 1 TBO 2 TBO 3 L1L1 L2L2 L3L3 R Order received Q Order placed Order placed Order received IP Q Order placed Q Order received Order received 0 IP Figure 12.7 – Q System When Demand Is Uncertain

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12 – 31 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Reorder Point 1.Choose an appropriate service-level policy Select service level or cycle service level Protection interval 2.Determine the demand during lead time probability distribution 3.Determine the safety stock and reorder point levels

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12 – 32 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Demand During Lead Time Specify mean and standard deviation Standard deviation of demand during lead time σ dLT = σ d 2 L = σ d L Safety stock and reorder point Safety stock = z σ dLT where z =number of standard deviations needed to achieve the cycle-service level σ dLT =stand deviation of demand during lead time Reorder point = R = dL + safety stock

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12 – 33 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Demand During Lead Time Example: Average demand is 75 per week Standard Deviation of Demand is 15 Lead Time is 3 weeks Desire an 85% service cycle level σ dLT = σ d 2 L = σ d L Safety stock = z σ dLT where z =number of standard deviations needed to achieve the cycle-service level σ dLT =stand deviation of demand during lead time Reorder point = R = dL + safety stock

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12 – 34 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. σ t = Demand for week 1 σ t = Demand for 3-week lead time + 75 Demand for week 2 σ t = 15 = 75 Demand for week 3 σ t = 15 Demand During Lead Time Figure 12.8 –Development of Demand Distribution for the Lead Time σ dLT = σ d 2 L = σ d L σ dLT = 15 3 σ dLT = 25.98

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12 – 35 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Demand During Lead Time Average demand during lead time Cycle-service level = 85% Probability of stockout (1.0 – 0.85 = 0.15) z σ dLT R a. Finding Safety Stock with a Normal Probability Distribution for an 85 Percent Cycle-Service Level: 85% : Z = 1.04 Safety stock = z σ dLT = 1.04 (25.98) = = 27 R = dL + safety stock = 75(3) + 27 = = 252 b. Determine Reorder Point:

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12 – 36 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Reorder Point for Variable Demand EXAMPLE 12.4 Let us return to the bird feeder in Example The EOQ is 75 units. Suppose that the average demand is 18 units per week with a standard deviation of 5 units. The lead time is constant at two weeks. Determine the safety stock and reorder point if management wants a 90 percent cycle- service level.

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12 – 37 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Reorder Point for Variable Demand SOLUTION In this case, σ d = 5, d = 18 units, and L = 2 weeks, so σ dLT = σ d L = 5 2 = Consult the body of the table in the Normal Distribution appendix for , which corresponds to a 90 percent cycle-service level. The closest number is , which corresponds to 1.2 in the row heading and 0.08 in the column heading. Adding these values gives a z value of With this information, we calculate the safety stock and reorder point as follows: Safety stock = z σ dLT = 1.28(7.07) = 9.05 or 9 units 2(18) + 9 = 45 units Reorder point= dL + Safety stock =

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12 – 38 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Application 12.3 Suppose that the demand during lead time is normally distributed with an average of 85 and σ dLT = 40. Find the safety stock, and reorder point R, for a 95 percent cycle-service level. SOLUTION Safety stock = z σ dLT = Find the safety stock, and reorder point R, for an 85 percent cycle-service level. R = Average demand during lead time + Safety stock R = = 151 units 1.645(40) = 65.8 or 66 units Safety stock = z σ dLT =1.04(40) = 41.6 or 42 units R = Average demand during lead time + Safety stock R = = 127 units

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12 – 39 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Reorder Point for Variable Demand and Lead Time Often the case that both are variable The equations are more complicated Safety stock = z σ dLT where d =Average weekly (or daily or monthly) demand L =Average lead time σ d =Standard deviation of weekly (or daily or monthly) demand σ LT =Standard deviation of the lead time σ dLT = Lσ d 2 + d 2 σ LT 2 R =(Average weekly demand Average lead time) + Safety stock = dL + Safety stock

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12 – 40 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Reorder Point EXAMPLE 12.5 The Office Supply Shop estimates that the average demand for a popular ball-point pen is 12,000 pens per week with a standard deviation of 3,000 pens. The current inventory policy calls for replenishment orders of 156,000 pens. The average lead time from the distributor is 5 weeks, with a standard deviation of 2 weeks. If management wants a 95 percent cycle- service level, what should the reorder point be?

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12 – 41 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Reorder Point SOLUTION From the Normal Distribution appendix for , the appropriate z value = We calculate the safety stock and reorder point as follows: σ dLT = Lσ d 2 + d 2 σ LT 2 = We have d = 12,000 pens, σ d = 3,000 pens, L = 5 weeks, and σ LT = 2 weeks (5)(3,000) 2 + (12,000) 2 (2) 2 = 24, pens Safety stock = z σ dLT = Reorder point = dL + Safety stock= (1.65)(24,919.87) =41, or 41,118 pens (12,000)(5) =101,118 pens

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12 – 42 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Application 12.4 Grey Wolf lodge is a popular 500-room hotel in the North Woods. Managers need to keep close tabs on all of the room service items, including a special pint-scented bar soap. The daily demand for the soap is 275 bars, with a standard deviation of 30 bars. Ordering cost is $10 and the inventory holding cost is $0.30/bar/year. The lead time from the supplier is 5 days, with a standard deviation of 1 day. The lodge is open 365 days a year. What should the reorder point be for the bar of soap if management wants to have a 99 percent cycle-service?

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12 – 43 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Application 12.4 SOLUTION d =275 bars L =5 days σ d =30 bars σ LT =1 day barsσ dLT = Lσ d 2 + d 2 σ LT 2 = From the Normal Distribution appendix for , z = We calculate the safety stock and reorder point as follows; Safety stock = z σ dLT = Reorder point + safety stock= dL + safety stock (2.33)(283.06) = or 660 bars = (275)(5) = 2,035 bars

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12 – 44 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Continuous Review Systems Two-Bin system Visual system An empty first bin signals the need to place an order Calculating total systems costs Total cost =Annual cycle inventory holding cost + Annual ordering cost + Annual safety stock holding cost C = ( H ) + ( S ) + ( H ) (Safety stock) Q2Q2 DQDQ

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12 – 45 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Application 12.5 The Discount Appliance Store uses a continuous review system ( Q system). One of the companys items has the following characteristics: Demand = 10 units/wk (assume 52 weeks per year) Ordering and setup cost ( S ) = $45/order Holding cost ( H ) = $12/unit/year Lead time ( L ) = 3 weeks (constant) Standard deviation in weekly demand = 8 units Cycle-service level = 70%

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12 – 46 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Application 12.5 SOLUTION What is the EOQ for this item? D = 10/wk 52 wks/yr = 520 units EOQ = = 2 DS H = 62 units 2(520)(45) 12 What is the desired safety stock? σ dLT = σ d L = 8 3 = 14 units Safety stock = z σ dLT = 0.525(14) = 8 units

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12 – 47 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Application 12.5 What is the desired reorder point R ? R = Average demand during lead time + Safety stock R = What is the total annual cost? 3(10) + 8 = 38 units ($12) + ($45) + 8($12) = $ C =

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12 – 48 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Application 12.5 Suppose that the current policy is Q = 80 and R = 150. What will be the changes in average cycle inventory and safety stock if your EOQ and R values are implemented? Reducing Q from 80 to 62 Cycle inventory reduction = 40 – 31 = 9 units Safety stock reduction = 120 – 8 = 112 units Reducing R from 150 to 38

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12 – 49 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Periodic Review System ( P ) Fixed interval reorder system or periodic reorder system Four of the original EOQ assumptions maintained No constraints are placed on lot size Holding and ordering costs Independent demand Lead times are certain Order is placed to bring the inventory position up to the target inventory level, T, when the predetermined time, P, has elapsed

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12 – 50 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Periodic Review System ( P ) PP T LLL Protection interval Time On-hand inventory IP 3 IP 1 IP 2 Order placed Order placed Order placed Order received Order received Order received IP OH Q1Q1 Q2Q2 Q3Q3 Figure – P System When Demand Is Uncertain

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12 – 51 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. How Much to Order in a P System EXAMPLE 12.6 A distribution center has a backorder for five 36-inch color TV sets. No inventory is currently on hand, and now is the time to review. How many should be reordered if T = 400 and no receipts are scheduled? SOLUTION IP = OH + SR – BO That is, 405 sets must be ordered to bring the inventory position up to T sets. = – 5 = –5 sets T – IP =400 – (–5) = 405 sets

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12 – 52 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Application 12.6 The on-hand inventory is 10 units, and T is 400. There are no back orders, but one scheduled receipt of 200 units. Now is the time to review. How much should be reordered? SOLUTION IP = OH + SR – BO The decision is to order 190 units = – 0 = 210 T – IP =400 – 210 = 190

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12 – 53 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Periodic Review System Selecting the time between reviews, choosing P and T T = d ( P + L ) + safety stock for protection interval Selecting T when demand is variable and lead time is constant IP covers demand over a protection interval of P + L The average demand during the protection interval is d ( P + L ), or Safety stock = zσ P + L, where σ P + L =

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12 – 54 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Calculating P and T EXAMPLE 12.7 Again, let us return to the bird feeder example. Recall that demand for the bird feeder is normally distributed with a mean of 18 units per week and a standard deviation in weekly demand of 5 units. The lead time is 2 weeks, and the business operates 52 weeks per year. The Q system developed in Example 12.4 called for an EOQ of 75 units and a safety stock of 9 units for a cycle-service level of 90 percent. What is the equivalent P system? Answers are to be rounded to the nearest integer.

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12 – 55 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Calculating P and T SOLUTION We first define D and then P. Here, P is the time between reviews, expressed in weeks because the data are expressed as demand per week: D = (18 units/week)(52 weeks/year) = 936 units P = (52) = EOQ D (52) = 4.2 or 4 weeks With d = 18 units per week, an alternative approach is to calculate P by dividing the EOQ by d to get 75/18 = 4.2 or 4 weeks. Either way, we would review the bird feeder inventory every 4 weeks.

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12 – 56 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Calculating P and T We now find the standard deviation of demand over the protection interval ( P + L ) = 6: Before calculating T, we also need a z value. For a 90 percent cycle-service level z = The safety stock becomes Safety stock = zσ P + L = 1.28(12.25) = or 16 units We now solve for T : = (18 units/week)(6 weeks) + 16 units = 124 units T = Average demand during the protection interval + Safety stock = d ( P + L ) + safety stock

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12 – 57 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Periodic Review System Use simulation when both demand and lead time are variable Suitable to single-bin systems Total costs for the P system are the sum of the same three cost elements as in the Q system Order quantity and safety stock are calculated differently C = ( H ) + ( S ) + Hzσ P + L dP 2 D dP

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12 – 58 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Application 12.7 Return to Discount Appliance Store (Application 12.4), but now use the P system for the item. Previous information Demand = 10 units/wk (assume 52 weeks per year) = 520 EOQ = 62 units (with reorder point system) Lead time ( L ) = 3 weeks Standard deviation in weekly demand = 8 units z = (for cycle-service level of 70%) Reorder interval P, if you make the average lot size using the Periodic Review System approximate the EOQ.

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12 – 59 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Application 12.7 SOLUTION Reorder interval P, if you make the average lot size using the Periodic Review System approximate the EOQ. P = (EOQ/ D )(52) = (62/529)(52) = 6.2 or 6 weeks Safety stock Target inventory T = 10(6 + 3) + 13 = 103 units T = d ( P + L ) + safety stock for protection interval Safety stock =

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12 – 60 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Application 12.7 Total cost C = ( H ) + ( S ) + Hzσ P + L dP 2 D dP = ($12) + ($45) + (13)($12) = $ (6) (6)

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12 – 61 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Comparative Advantages Primary advantages of P systems Convenient Orders can be combined Only need to know IP when review is made Primary advantages of Q systems Review frequency may be individualized Fixed lot sizes can result in quantity discounts Lower safety stocks

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12 – 62 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Optional replenishment systems Optimal review, min-max, or ( s, S ) system, like the P system Reviews IP at fixed time intervals and places a variable- sized order to cover expected needs Ensures that a reasonable large order is placed Hybrid systems Base-stock system Replenishment order is issued each time a withdrawal is made Order quantities vary to keep the inventory position at R Minimizes cycle inventory, but increases ordering costs Appropriate for expensive items

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12 – 63 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Solved Problem 1 Bookers Book Bindery divides SKUs into three classes, according to their dollar usage. Calculate the usage values of the following SKUs and determine which is most likely to be classified as class A. SKU NumberDescription Quantity Used per Year Unit Value ($) 1Boxes Cardboard (square feet) 18, Cover stock10, Glue (gallons) Inside covers20, Reinforcing tape (meters) 3, Signatures150,

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12 – 64 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Solved Problem 1 SOLUTION The annual dollar usage for each item is determined by multiplying the annual usage quantity by the value per unit. As shown in Figure 12.11, the SKUs are then sorted by annual dollar usage, in declining order. Finally, A–B and B–C class lines are drawn roughly, according to the guidelines presented in the text. Here, class A includes only one SKU (signatures), which represents only 1/7, or 14 percent, of the SKUs but accounts for 83 percent of annual dollar usage. Class B includes the next two SKUs, which taken together represent 28 percent of the SKUs and account for 13 percent of annual dollar usage. The final four SKUs, class C, represent over half the number of SKUs but only 4 percent of total annual dollar usage.

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12 – 65 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Solved Problem 1 SKU Number Description Quantity Used per Year Unit Value ($) Annual Dollar Usage ($) 1Boxes =1,500 2Cardboard (square feet) 18, =360 3Cover stock10, =7,500 4Glue (gallons) =3,000 5Inside covers20, =1,000 6Reinforcing tape (meters) 3, =450 7Signatures150, =67,500 Total81,310

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12 – 66 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Solved Problem 1 Percentage of SKUs Percentage of Dollar Value 100 – 90 – 80 – 70 – 60 – 50 – 40 – 30 – 20 – 10 – 0 – Class C Class A Class B Figure –Annual Dollar Usage for Class A, B, and C SKUs Using Tutor 12.2

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12 – 67 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Solved Problem 2 Nelsons Hardware Store stocks a 19.2 volt cordless drill that is a popular seller. Annual demand is 5,000 units, the ordering cost is $15, and the inventory holding cost is $4/unit/year. a.What is the economic order quantity? b.What is the total annual cost for this inventory item? SOLUTION a. The order quantity is EOQ = = 2 DS H 2(5,000)($15) $4 = 37,500 = or 194 drills b.The total annual cost is C = ( H ) + ( S ) = Q2Q2 DQDQ ($4) + ($15) = $ ,

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12 – 68 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Solved Problem 3 A regional distributor purchases discontinued appliances from various suppliers and then sells them on demand to retailers in the region. The distributor operates 5 days per week, 52 weeks per year. Only when it is open for business can orders be received. Management wants to reevaluate its current inventory policy, which calls for order quantities of 440 counter-top mixers. The following data are estimated for the mixer: Average daily demand ( d ) = 100 mixers Standard deviation of daily demand ( σ d ) = 30 mixers Lead time ( L ) = 3 days Holding cost ( H ) = $9.40/unit/year Ordering cost ( S ) = $35/order Cycle-service level = 92 percent The distributor uses a continuous review ( Q ) system

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12 – 69 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Solved Problem 3 a.What order quantity Q, and reorder point, R, should be used? b.What is the total annual cost of the system? c.If on-hand inventory is 40 units, one open order for 440 mixers is pending, and no backorders exist, should a new order be placed?

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12 – 70 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Solved Problem 3 SOLUTION a.Annual demand is The order quantity is D = (5 days/week)(52 weeks/year)(100 mixers/day) = 26,000 mixers/year EOQ = = 2 DS H 2(26,000)($35) $9.40 = 193,167 = or 440 mixers

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12 – 71 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Solved Problem 3 The standard deviation of the demand during lead time distribution is A 92 percent cycle-service level corresponds to z = 1.41 σ dLT = σ d L = 30 3 = Safety stock = z σ dLT = 1.41(51.96 mixers) = or 73 mixers 100(3) = 300 mixersAverage demand during lead time = dL = Reorder point ( R ) = Average demand during lead time + Safety stock = 300 mixers + 73 mixers = 373 mixers With a continuous review system, Q = 440 and R = 373

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12 – 72 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Solved Problem 3 b.The total annual cost for the Q systems is C = ( H ) + ( S ) + ( H )(Safety stock) Q2Q2 DQDQ C = ($9.40) + ($35) + ($9.40)(73) = $4, , c.Inventory position =On-hand inventory + Scheduled receipts – Backorders IP = OH + SR – BO = – 0 = 480 mixers Because IP (480) exceeds R (373), do not place a new order

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12 – 73 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Solved Problem 4 Suppose that a periodic review ( P ) system is used at the distributor in Solved Problem 3, but otherwise the data are the same. a.Calculate the P (in workdays, rounded to the nearest day) that gives approximately the same number of orders per year as the EOQ. b.What is the target inventory level, T ? Compare the P system to the Q system in Solved Problem 3. c.What is the total annual cost of the P system? d.It is time to review the item. On-hand inventory is 40 mixers; receipt of 440 mixers is scheduled, and no backorders exist. How much should be reordered?

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12 – 74 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Solved Problem 4 SOLUTION a. The time between orders is P = (260 days/year) = EOQ D (260) = 4.4 or 4 days ,000 b.Figure shows that T = 812 and safety stock = (1.41)(79.37) = or about 112 mixers. The corresponding Q system for the counter-top mixer requires less safety stock. Figure – OM Explorer Solver for Inventory Systems

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12 – 75 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Solved Problem 4 c.The total annual cost of the P system is C = ( H ) + ( S ) + ( H )(Safety stock) dP 2 D dP C = ($9.40) + ($35) + ($9.40)(1.41)(79.37) 100(4) 2 26, (4) = $5, d.Inventory position is the amount on hand plus scheduled receipts minus backorders, or IP = OH + SR – BO = – 0 = 480 mixers The order quantity is the target inventory level minus the inventory position, or Q = T – IP = An order for 332 mixers should be placed. 812 mixers – 480 mixers = 332 mixers

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12 – 76 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Solved Problem 5 Grey Wolf Lodge is a popular 500-room hotel in the North Woods. Managers need to keep close tabs on all room service items, including a special pine-scented bar soap. The daily demand for the soap is 275 bars, with a standard deviation of 30 bars. Ordering cost is $10 and the inventory holding cost is $0.30/bar/year. The lead time from the supplier is 5 days, with a standard deviation of 1 day. The lodge is open 365 days a year. a.What is the economic order quantity for the bar of soap? b.What should the reorder point be for the bar of soap if management wants to have a 99 percent cycle-service level? c.What is the total annual cost for the bar of soap, assuming a Q system will be used?

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12 – 77 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Solved Problem 5 SOLUTION a.We have D = (275)(365) = 100,375 bars of soap; S = $10; and H = $0.30. The EOQ for the bar of soap is EOQ = = 2 DS H 2(100,375)($10) $0.30 = 6,691,666.7 = 2, or 2,587 bars

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12 – 78 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Solved Problem 5 b.We have d = 275 bars/day, σ d = 30 bars, L = 5 days, and σ LT = 1 day. σ dLT = Lσ d 2 + d 2 σ LT 2 = (5)(30) 2 + (275) 2 (1) 2 = bars Consult the body of the Normal Distribution appendix for The closest value is , which corresponds to a z value of We calculate the safety stock and reorder point as follows: Safety stock = z σ dLT = (2.33)(283.06) = or 660 bars Reorder point = dL + Safety stock= (275)(5) = 2,035 bars

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12 – 79 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Solved Problem 5 c.The total annual cost for the Q system is C = ( H ) + ( S ) + ( H )(Safety stock) Q2Q2 DQDQ C = ($0.30) + ($10) + ($0.30)(660) = $ , ,375 2,587

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12 – 80 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Solved Problem 6 Zekes Hardware Store sells furnace filters. The cost to place an order to the distributor is $25 and the annual cost to hold a filter in stock is $2. The average demand per week for the filters is 32 units, and the store operates 50 weeks per year. The weekly demand for filters has the probability distribution shown on the left below. The delivery lead time from the distributor is uncertain and has the probability distribution shown on the right below. Suppose Zeke wants to use a P system with P = 6 weeks and a cycle-service level of 90 percent. What is the appropriate value for T and the associated annual cost of the system?

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12 – 81 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Solved Problem 6 DemandProbability Lead Time (wks)Probability

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12 – 82 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Solved Problem 6 SOLUTION Figure contains output from the Demand During the Protection Interval Simulator from OM Explorer. Figure – OM Explorer Solver for Demand during the Protection Interval

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12 – 83 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Solved Problem 6 Given the desired cycle-service level of 90 percent, the appropriate T value is 322 units. The simulation estimated the average demand during the protection interval to be 289 units, consequently the safety stock is 322 – 289 = 33 units. The annual cost of this P system is C = ($2) + ($25) + (33)($2) 6(32) 2 50(32) 6(32) = $ $ $66.00 = $466.33

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12 – 84 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Solved Problem 7 Consider Zekes inventory in Solved Problem 6. Suppose that he wants to use a continuous review ( Q ) system for the filters, with an order quantity of 200 and a reorder point of 140. Initial inventory is 170 units. If the stockout cost is $5 per unit, and all of the other data in Solved Problem 6 are the same, what is the expected cost per week of using the Q system? SOLUTION Figure shows output from the Q System Simulator in OM Explorer. Only weeks 1 through 13 and weeks 41 through 50 are shown in the figure. The average total cost per week is $ Notice that no stockouts occurred in this simulation. These results are dependent on Zekes choices for the reorder point and lot size. It is possible that stockouts would occur if the simulation were run for more than 50 weeks.

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12 – 85 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall. Solved Problem 7 Figure – OM Explorer Q System Simulator

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12 – 86 Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall.

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