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- 1. 12 Inventory Management McGraw-Hill/Irwin Copyright © 2007 by The McGraw-Hill Companies, Inc. All rights reserved.
- 2. Learning Objectives Define the term inventory and list the major reasons for holding inventories; and list the main requirements for effective inventory management. Discuss the nature and importance of service inventories Discuss periodic and perpetual review systems. Discuss the objectives of inventory management. Describe the A-B-C approach and explain how it is useful. 12-2
- 3. Learning Objectives Describe the basic EOQ model and its assumptions and solve typical problems. Describe the economic production quantity model and solve typical problems. Describe the quantity discount model and solve typical problems. Describe reorder point models and solve typical problems. Describe situations in which the singleperiod model would be appropriate, and solve typical problems. 12-3
- 4. Inventory Inventory: a stock or store of goods Dependent Demand A C(2) B(4) D(2) Independent Demand E(1) D(3) F(2) Independent demand is uncertain. Dependent demand is certain. 12-4
- 5. Inventory Models Independent demand – finished goods, items that are ready to be sold E.g. a computer Dependent demand – components of finished products E.g. parts that make up the computer 12-5
- 6. Types of Inventories Raw materials & purchased parts Partially completed goods called work in progress Finished-goods inventories (manufacturing firms) or merchandise (retail stores) 12-6
- 7. Types of Inventories (Cont’d) Replacement parts, tools, & supplies Goods-in-transit to warehouses or customers 12-7
- 8. Functions of Inventory To meet anticipated demand To smooth production requirements (to build inventories during preseason period) To decouple operations (in terms of operational difficulties) To protect against stock-outs 12-8
- 9. Functions of Inventory (Cont’d) To take advantage of order cycles (to produce in economic lot size) To help hedge against price increases To permit operations (work in progress inventory) To take advantage of quantity discounts 12-9
- 10. Objective of Inventory Control To achieve satisfactory levels of customer service while keeping inventory costs within reasonable bounds Level of customer service Costs of ordering and carrying inventory Inventory turnover is the ratio of average cost of goods sold to average inventory investment. 12-10
- 11. Effective Inventory Management A system to keep track of inventory A reliable forecast of demand Knowledge of lead times Reasonable estimates of Holding costs Ordering costs Shortage costs A classification system 12-11
- 12. Inventory Counting Systems Periodic System Physical count of items made at periodic intervals (Manager periodically check the shelves) Perpetual Inventory System (Continuous) System that keeps track of removals from inventory continuously, thus monitoring current levels of each item 12-12
- 13. Inventory Counting Systems (Cont’d) Two-Bin System - Two containers of inventory; reorder when the first is empty Universal Bar Code - Bar code printed on a label that has information about the item to which it is attached 0 214800 232087768 12-13
- 14. Key Inventory Terms Lead time: time interval between ordering and receiving the order Holding (carrying) costs: cost to carry an item in inventory for a length of time, usually a year Ordering costs: costs of ordering and receiving inventory Shortage costs: costs when demand exceeds supply 12-14
- 15. ABC Classification System Figure 12.1 Classifying inventory according to some measure of importance and allocating control efforts accordingly. A - very important B - mod. important C - least important High Annual $ value of items A B C Low Low High Percentage of Items 12-15
- 16. Economic Order Quantity Models Economic order quantity (EOQ) model The order size that minimizes total annual cost Economic production model 12-16
- 17. Assumptions of EOQ Model Only one product is involved Annual demand requirements known Demand is even throughout the year Lead time does not vary Each order is received in a single delivery There are no quantity discounts 12-17
- 18. Figure 12.2 Q Quantity on hand The Inventory Cycle Profile of Inventory Level Over Time Usage rate Reorder point Receive order Place Receive order order Place Receive order order Time Lead time 12-18
- 19. Total Cost Annual Annual Total cost = carrying + ordering cost cost TC = Q H 2 + DS Q 12-19
- 20. Cost Minimization Goal Figure 12.4C Annual Cost The Total-Cost Curve is U-Shaped Q D TC = H + S 2 Q Ordering Costs QO (optimal order quantity) Order Quantity (Q) 12-20
- 21. Minimum Total Cost The total cost curve reaches its minimum where the carrying and ordering costs are equal. Q H 2 = DS Q 12-21
- 22. Deriving the EOQ Using calculus, we take the derivative of the total cost function and set the derivative (slope) equal to zero and solve for Q. Q OPT = 2DS = H 2(Annual Demand)(Order or Setup Cost) Annual Holding Cost 12-22
- 23. Example A local distributor for a national tire company expects to sell approximately 9,600 steel belted radial tires of a certain cize and tread design next year. Annual carrying cost is $16 per tire and ordering cost is $75. The distributor operates 288 days a year. a) What is the Economic Order Quantity? b) How many times per year does the store Re-order? c) What is the length of an order cycle? d) What is the total annual cost if the EOQ quantity is ordered? 12-23
- 24. Economic Production Quantity (EPQ) Production done in batches or lots Capacity to produce a part exceeds the part’s usage or demand rate Assumptions of EPQ are similar to EOQ except orders are received incrementally during production 12-24
- 25. Economic Production Quantity Assumptions Only one item is involved Annual demand is known Usage rate is constant Usage occurs continually Production rate is constant Lead time does not vary No quantity discounts 12-25
- 26. Economic Run Size Q0 = 2DS p H p− u 12-26
- 27. A toy Manufacturing uses 48,000 rubber weels per year for its popular dump truck series. The firm makes its own wheels, which it can produce at a rate of 800 per day. The toy trucks are assembled uniformly over the entire year. Carrying cost is $1 per wheel a year. Setup cost for a production run of a wheels is $45. The firm operates 240 days per year. Determine the :a) Optimal run size (optimal quantity produced) b) Minimum total annual cost for carrying cost and setup. c) Cycle time for the optimal run size? d) Run time 12-27
- 28. Quantity Figure 12.12 Safety Stock Maximum probable demand during lead time Expected demand during lead time ROP Safety stock reduces risk of stockout during lead time Safety stock LT Time 12-28
- 29. THANKS 12-29

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