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Inventory Management

Inventory Management. Types of Inventories. Raw materials & purchased parts Partially completed goods called work in progress Finished-goods inventories ( manufacturing firms ) or merchandise ( retail stores ). Types of Inventories (Cont’d). Replacement parts, tools, & supplies

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Inventory Management

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  1. Inventory Management

  2. Types of Inventories • Raw materials & purchased parts • Partially completed goods called work in progress • Finished-goods inventories • (manufacturingfirms) or merchandise (retail stores)

  3. Types of Inventories (Cont’d) • Replacement parts, tools, & supplies • Goods-in-transit to warehouses or customers

  4. Functions of Inventory • To meet anticipated demand • To smooth production requirements • To decouple components of the production-distribution • To protect against stock-outs

  5. Functions of Inventory (Cont’d) • To take advantage of order cycles • To help hedge against price increases or to take advantage of quantity discounts • To permit operations

  6. 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

  7. High A Annual $ volume of items B C Low Few Many Number of Items ABC Classification System Figure 13-1 Classifying inventory according to some measure of importance and allocating control efforts accordingly. A-very important B- mod. important C- least important

  8. Economic Order Quantity Models • Economic order quantity model • Economic production model • Quantity discount model

  9. 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

  10. Profile of Inventory Level Over Time Q Usage rate Quantity on hand Reorder point Time Place order Place order Receive order Receive order Receive order Lead time The Inventory Cycle Figure 13-2

  11. Annual carrying cost Annual ordering cost Total cost = + Q D S H TC = + 2 Q Total Cost

  12. The Total-Cost Curve is U-Shaped Annual Cost Ordering Costs Order Quantity (Q) QO (optimal order quantity) Cost Minimization Goal Figure 13-4

  13. Minimum Total Cost The total cost curve reaches its minimum where the carrying and ordering costs are equal.

  14. Economic Production Quantity

  15. 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

  16. Economic Production Quantity

  17. Annual carrying cost Annual ordering cost Purchasing cost + TC = + Q D PD S H TC = + + 2 Q Quantity Discounts

  18. Cost Adding Purchasing costdoesn’t change EOQ TC with PD TC without PD PD 0 Quantity EOQ Total Costs with PD Figure 13-7

  19. Total Cost with Constant Carrying Costs

  20. Total Cost with Constant Carrying Costs

  21. Total Cost with Variable Carrying Costs

  22. Total Cost with Variable Carrying Costs

  23. When to Reorder with EOQ Ordering • Reorder Point- When the quantity on hand of an item drops to this amount, the item is reordered • Safety Stock- Stock that is held in excess of expected demand due to variable demand rate and/or lead time. • Service Level- Probability that demand will not exceed supply during lead time. • SL=100-stockout risk

  24. Reorder Point (ROP)

  25. Reorder Point (ROP)

  26. Reorder Point (ROP)

  27. Shortage and Service Level

  28. Fixed-Order-Interval Model • Orders are placed at fixed time intervals • Order quantity for next interval? • Suppliers might encourage fixed intervals • May require only periodic checks of inventory levels

  29. Fixed-Order-Interval Model

  30. Stockout Risk

  31. Fixed-Interval Benefits • Tight control of type A items • Items from same supplier may yield savings in: • Ordering • Packing • Shipping costs • May be practical when inventories cannot be closely monitored

  32. Fixed-Interval Disadvantages • Requires a larger safety stock • Increases carrying cost • Costs of periodic reviews

  33. Single Period Model • Single period model: model for ordering of perishables and other items with limited useful lives • Shortage cost: generally the unrealized profits per unit • Excess cost: difference between purchase cost and salvage value of items left over at the end of a period

  34. Single Period Model • Cshortage = Cs = revenue per unit - cost per unit • Cexcess = Ce = original cost per unit - salvage value per unit

  35. Single Period Model • Continuous stocking levels • Identifies optimal stocking levels • Optimal stocking level balances unit shortage and excess cost • Discrete stocking levels • Service levels are discrete rather than continuous • Desired service level is equaled or exceeded

  36. Operations Strategy • Too much inventory • Tends to hide problems • Easier to live with problems than to eliminate them • Costly to maintain • Wise strategy • Reduce lot sizes • Reduce safety stock

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