MGT366 Ch12


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MGT366 Ch12

  1. 1. Chapter 12 Inventory Management McGraw-Hill/Irwin Copyright © 2009 by The McGraw-Hill Companies, Inc. All Rights Reserved.
  2. 2. Inventory <ul><li>Inventory </li></ul><ul><ul><li>A stock or store of goods </li></ul></ul><ul><li>Independent demand items </li></ul><ul><ul><li>Items that are ready to be sold or used </li></ul></ul>12-
  3. 3. Types of Inventory <ul><li>Raw materials and purchased parts </li></ul><ul><li>Work-in-process </li></ul><ul><li>Finished goods inventories or merchandise </li></ul><ul><li>Maintenance and repairs (MRO) inventory, tools and supplies </li></ul><ul><li>Goods-in-transit to warehouses or customers (pipeline inventory) </li></ul>12-
  4. 4. Inventory Functions <ul><li>Inventories serve a number of functions such as: </li></ul><ul><ul><li>To meet anticipated customer demand </li></ul></ul><ul><ul><li>To smooth production requirements </li></ul></ul><ul><ul><li>To decouple operations </li></ul></ul><ul><ul><li>To protect against stockouts </li></ul></ul><ul><ul><li>To take advantage of order cycles </li></ul></ul><ul><ul><li>To hedge against price increases </li></ul></ul><ul><ul><li>To permit operations </li></ul></ul><ul><ul><li>To take advantage of quantity discounts </li></ul></ul>12-
  5. 5. Inventory Management <ul><li>Management has two basic functions concerning inventory: </li></ul><ul><ul><li>Establish a system for tracking items in inventory </li></ul></ul><ul><ul><li>Make decisions about </li></ul></ul><ul><ul><ul><li>When to order </li></ul></ul></ul><ul><ul><ul><li>How much to order </li></ul></ul></ul>12-
  6. 6. Effective Inventory Management <ul><li>Requires: </li></ul><ul><ul><li>A system keep track of inventory </li></ul></ul><ul><ul><li>A reliable forecast of demand </li></ul></ul><ul><ul><li>Knowledge of lead time and lead time variability </li></ul></ul><ul><ul><li>Reasonable estimates of </li></ul></ul><ul><ul><ul><li>holding costs </li></ul></ul></ul><ul><ul><ul><li>ordering costs </li></ul></ul></ul><ul><ul><ul><li>shortage costs </li></ul></ul></ul><ul><ul><li>A classification system for inventory items </li></ul></ul>12-
  7. 7. Inventory Counting Systems <ul><li>Periodic System </li></ul><ul><ul><li>Physical count of items in inventory made at periodic intervals </li></ul></ul><ul><li>Perpetual Inventory System </li></ul><ul><ul><li>System that keeps track of removals from inventory continuously, thus monitoring current levels of each item </li></ul></ul><ul><ul><ul><li>Two-bin system </li></ul></ul></ul><ul><ul><ul><ul><li>Two containers of inventory; reorder </li></ul></ul></ul></ul><ul><ul><ul><ul><li>when the first is empty </li></ul></ul></ul></ul>12-
  8. 8. Inventory Counting Technologies <ul><li>Universal product code (UPC) </li></ul><ul><ul><li>Bar code printed on a label that has information about the item to which it is attached </li></ul></ul><ul><li>Radio frequency identification (RFID) tags </li></ul><ul><ul><li>A technology that uses radio waves to identify objects, such as goods in supply chains </li></ul></ul>12-
  9. 9. Demand Forecasts and Lead Time <ul><li>Forecasts </li></ul><ul><ul><li>Inventories are necessary to satisfy customer demands, so it is important to have a reliable estimates of the amount and timing of demand </li></ul></ul><ul><li>Lead time </li></ul><ul><ul><li>Time interval between ordering and receiving the order </li></ul></ul><ul><li>Point-of-sale (POS) systems </li></ul><ul><ul><li>A system that electronically records actual sales </li></ul></ul><ul><ul><li>Such demand information is very useful for enhancing forecasting and inventory management </li></ul></ul>12-
  10. 10. ABC Classification System <ul><li>A-B-C approach </li></ul><ul><ul><li>Classifying inventory according to some measure of importance, and allocating control efforts accordingly </li></ul></ul><ul><ul><li>A items (very important) </li></ul></ul><ul><ul><ul><li>10 to 20 percent of the number of items in inventory and about 60 to 70 percent of the annual dollar value </li></ul></ul></ul><ul><ul><li>B items (moderately important) </li></ul></ul><ul><ul><li>C items (least important) </li></ul></ul><ul><ul><ul><li>50 to 60 percent of the number </li></ul></ul></ul><ul><ul><ul><li>of items in inventory but only </li></ul></ul></ul><ul><ul><ul><li>about 10 to 15 percent of the </li></ul></ul></ul><ul><ul><ul><li>annual dollar value </li></ul></ul></ul>12- Annual $ value of items High Low Few Many Number of Items A C B
  11. 11. Cycle Counting <ul><li>Cycle counting </li></ul><ul><ul><li>A physical count of items in inventory </li></ul></ul><ul><li>Cycle counting management </li></ul><ul><ul><li>How much accuracy is needed? </li></ul></ul><ul><ul><ul><li>A items: ± 0.2 percent </li></ul></ul></ul><ul><ul><ul><li>B items: ± 1 percent </li></ul></ul></ul><ul><ul><ul><li>C items: ± 5 percent </li></ul></ul></ul><ul><ul><li>When should cycle counting be performed? </li></ul></ul><ul><ul><li>Who should do it? </li></ul></ul>12-
  12. 12. How Much to Order: EOQ Models <ul><li>The basic economic order quantity model </li></ul><ul><li>The economic production quantity model </li></ul><ul><li>The quantity discount model </li></ul>12-
  13. 13. Basic EOQ Model <ul><li>The basic EOQ model is used to find a fixed order quantity that will minimize total annual inventory costs </li></ul><ul><li>Assumptions </li></ul><ul><ul><li>Only one product is involved </li></ul></ul><ul><ul><li>Annual demand requirements are known </li></ul></ul><ul><ul><li>Demand is even throughout the year </li></ul></ul><ul><ul><li>Lead time does not vary </li></ul></ul><ul><ul><li>Each order is received in a single delivery </li></ul></ul><ul><ul><li>There are no quantity discounts </li></ul></ul>12-
  14. 14. The Inventory Cycle 12- Profile of Inventory Level Over Time Quantity on hand Q Receive order Place order Receive order Place order Receive order Lead time Reorder point Usage rate Time
  15. 15. Total Annual Cost 12-
  16. 16. Goal: Total Cost Minimization 12- Order Quantity (Q) The Total-Cost Curve is U-Shaped Ordering Costs Q * Annual Cost ( optimal order quantity) Holding Costs
  17. 17. Deriving EOQ <ul><li>Using calculus, we take the derivative of the total cost function and set the derivative (slope) equal to zero and solve for Q. </li></ul><ul><li>The total cost curve reaches its minimum where the carrying and ordering costs are equal. </li></ul>12-
  18. 18. Economic Production Quantity (EPQ) <ul><li>Assumptions </li></ul><ul><ul><li>Only one product is involved </li></ul></ul><ul><ul><li>Annual demand requirements are known </li></ul></ul><ul><ul><li>Usage rate is constant </li></ul></ul><ul><ul><li>Usage occurs continually, but production occurs periodically </li></ul></ul><ul><ul><li>The production rate is constant </li></ul></ul><ul><ul><li>Lead time does not vary </li></ul></ul><ul><ul><li>There are no quantity discounts </li></ul></ul>12-
  19. 19. EPQ: Inventory Profile 12- Q Q * I max Production and usage Production and usage Production and usage Usage only Usage only Cumulative production Amount on hand Time
  20. 20. EPQ – Total Cost 12-
  21. 21. EPQ 12-
  22. 22. Quantity Discount Model <ul><li>Quantity discount </li></ul><ul><ul><li>Price reduction offered to customers for placing large orders </li></ul></ul>12-
  23. 23. Quantity Discounts 12-
  24. 24. Quantity Discounts 12-
  25. 25. When to Reorder <ul><li>Reorder point </li></ul><ul><ul><li>When the quantity on hand of an item drops to this amount, the item is reordered. </li></ul></ul><ul><ul><li>Determinants of the reorder point </li></ul></ul><ul><ul><ul><li>The rate of demand </li></ul></ul></ul><ul><ul><ul><li>The lead time </li></ul></ul></ul><ul><ul><ul><li>The extent of demand and/or lead time variability </li></ul></ul></ul><ul><ul><ul><li>The degree of stockout risk acceptable to management </li></ul></ul></ul>12-
  26. 26. Reorder Point: Under Certainty 12-
  27. 27. Reorder Point: Under Uncertainty <ul><li>Demand or lead time uncertainty creates the possibility that demand will be greater than available supply </li></ul><ul><li>To reduce the likelihood of a stockout, it becomes necessary to carry safety stock </li></ul><ul><ul><li>Safety stock </li></ul></ul><ul><ul><ul><li>Stock that is held in excess of expected demand due to variable demand and/or lead time </li></ul></ul></ul>12-
  28. 28. Safety Stock 12- LT Time Expected demand during lead time Maximum probable demand during lead time ROP Quantity Safety stock
  29. 29. Safety Stock? <ul><li>As the amount of safety stock carried increases, the risk of stockout decreases. </li></ul><ul><ul><li>This improves customer service level </li></ul></ul><ul><li>Service level </li></ul><ul><ul><li>The probability that demand will not exceed supply during lead time </li></ul></ul><ul><ul><li>Service level = 100% - Stockout risk </li></ul></ul>12-
  30. 30. How Much Safety Stock? <ul><li>The amount of safety stock that is appropriate for a given situation depends upon: </li></ul><ul><ul><li>The average demand rate and average lead time </li></ul></ul><ul><ul><li>Demand and lead time variability </li></ul></ul><ul><ul><li>The desired service level </li></ul></ul>12-
  31. 31. Reorder Point ROP Risk of stockout Service level Expected demand Safety stock 0 z Quantity z-scale The ROP based on a normal Distribution of lead time demand 12-
  32. 32. Reorder Point: Demand Uncertainty 12-
  33. 33. Reorder Point: Lead Time Uncertainty 12-
  34. 34. How Much to Order: FOI <ul><li>Fixed-order-interval (FOI) model </li></ul><ul><ul><li>Orders are placed at fixed time intervals </li></ul></ul><ul><li>Reasons for using the FOI model </li></ul><ul><ul><li>Supplier’s policy may encourage its use </li></ul></ul><ul><ul><li>Grouping orders from the same supplier can produce savings in shipping costs </li></ul></ul><ul><ul><li>Some circumstances do not lend themselves to continuously monitoring inventory position </li></ul></ul>12-
  35. 35. Fixed-Quantity vs. Fixed-Interval Ordering 12-
  36. 36. FOI Model 12-
  37. 37. FOI Model OI * represents the optimal time between orders. Time-frame of interest is an appropriate period (e.g., days or weeks). This is usually based on the time-frame expressed by the average demand rate, d-bar. 12-
  38. 38. Single-Period Model <ul><li>Single-period model </li></ul><ul><ul><li>Model for ordering perishables and other items with limited useful lives </li></ul></ul><ul><ul><li>Shortage cost </li></ul></ul><ul><ul><ul><li>Generally, the unrealized profit per unit </li></ul></ul></ul><ul><ul><ul><li>C shortage = C s = Revenue per unit – Cost per unit </li></ul></ul></ul><ul><ul><li>Excess cost </li></ul></ul><ul><ul><ul><li>Different between purchase cost and salvage value of items left over at the end of the period </li></ul></ul></ul><ul><ul><ul><li>C excess = C e = Cost per unit – Salvage value per unit </li></ul></ul></ul>12-
  39. 39. Single-Period Model <ul><li>The goal of the single-period model is to identify the order quantity that will minimize the long-run excess and shortage costs </li></ul><ul><li>Two categories of problem: </li></ul><ul><ul><li>Demand can be characterized by a continuous distribution </li></ul></ul><ul><ul><li>Demand can be characterized by a discrete distribution </li></ul></ul>12-
  40. 40. Stocking Levels 12- Service level S o Balance Point Quantity C s C e S o =Optimum Stocking Quantity