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BUAD 306 Chapter 13 - Inventory Management
Everyday Inventory Food ¢ Gasoline ¢ Clean clothes… ¢ What else?
Inventory ¢ ¢ Stock or quantity of items kept to meet demand Takes on different forms l l l Final goods Raw materials Purchased/component parts Labor In-process materials Working capital
Inventory Static – only one opportunity to buy and sell units ¢ Dynamic – ongoing need for units; reordering must take place ¢
Demand ¢ Dependent Demand l ¢ Items are used internally to produce a final product Independent Demand l Items are final products demanded by external customers
Reasons To Hold Inventory ¢ ¢ ¢ ¢ To meet anticipated demand To smooth production requirements To decouple components of the productiondistribution system To protect against stock-outs To take advantage of order cycles To hedge against price increases or to take advantage of quantity discounts To permit operations
Inventory Costs Carrying Costs - Storage, warehousing, insurance, security, taxes, opportunity cost, depreciation, etc. ¢ Ordering Costs - Determining quantities needed, preparing documentation, shipping, inspection of goods, etc. ¢ Stockout Costs – Temporary or permanent loss of sales / goodwill when demand cannot be met ¢
Inventory Management How much and when to order inventory? Objective: To keep enough inventory to meet customer demand AND also be cost-effective ¢ Goal: To determine the amount of inventory to keep in stock - how much to order AND when to order ¢
Inventory Management Requirements A system to keep track of the inventory on hand on order ¢ A reliable forecast of demand ¢ Knowledge of lead times ¢ Reasonable estimates of inventory costs ¢ A classification system for inventory items (ABC) ¢
Inventory Control Systems ¢ Control the level of inventory by determining how much to order and when Continuous (Perpetual) Inventory System - a continual record of the inventory level for every item is maintained l Periodic Inventory System - inventory on hand is counted at specific time intervals l
Other Control Systems/Tools Universal Product Codes (UPC) ¢ RFID Tags ¢ Two-Bin System – two containers of inventory; reorder when the first is empty ¢ 0 214800 232087768
Considerations ¢ Lead Time l ¢ Cycle Counting l ¢ Time interval between ordering and receiving the order Physical count of items in inventory Usage Rate l Rate at which amount of inventory is depleted
Inventory Cycle Q Profile of Inventory Level Over Time Usage rate Quantity on hand Reorder point Receive order Place Receive order Lead time Place order Receive order Time
Economic Order Quantity ¢ The EOQ Model determines the optimal order size that minimizes total inventory costs
Optimal Order Quantity Q o = 2 DS = 2 (Annual Demand) (Order Cost) H Annual Holding Cost per unit Length of order cycle = Qo D # Orders / Year = D Qo
Basic EOQ Model Annual Total cost = carrying + ordering cost TC = Where: Qo H 2 + DS Qo Qo = Economic order quantity in units H = Holding (carrying) cost per unit D = Demand, usually in units per year S = Ordering cost
Cost Minimization Goal Annual Cost The Total-Cost Curve is U-Shaped Carrying Costs Ordering Costs QO (optimal order quantity) Order Quantity (Q)
EOQ Example 1 A local office supply store expects to sell 2400 printers next year. Annual carrying cost is $50 per printer, and ordering cost is $30. The company operates 300 days a year. A) What is the EOQ? B) How many times per year does the store reorder? C) What is the length of an order cycle? D) What is the total annual cost if the EOQ quantity is ordered?
EOQ Example 2 A local electronics store expects to sell 500 flat-screen TVs each month during next year. Annual carrying cost is $60 per TV, and ordering cost is $50. The company operates 364 days a year. A) What is the EOQ? B) How many times per year does the store reorder? C) What is the length of an order cycle? D) What is the total annual cost if the EOQ quantity is ordered?
Quantity Discounts ¢A price discount on an item if predetermined numbers of units are ordered TC = Carrying cost + Ordering cost + Purchasing cost = (Q / 2) H + (D / Q) S + PD where P = Unit Price
Quantity Discount Example Campus Computers 2 Go Inc. wants to reduce a large stock of laptops it is discontinuing. It has offered the University Bookstore a quantity discount pricing schedule as shown below. Given the discount schedule and its known costs, the bookstore wants to determine if it should take advantage of this discount or order the basic EOQ order size. Quantity Price Carrying Cost: $200 1 – 49 $1, 500 Ordering Cost $1, 000 50 – 89 $1, 000 Annual Demand 400 units 90 + $800
EPQ – Economic Production Quantity (OQ with Incremental Replenishment) Used when company makes its own product ¢ Considers a variety of costs/terms: ¢ Carrying Cost l Setup Cost (analogous to ordering costs) l Maximum and Average Inventory Levels l Economic Run Quantity l Cycle Time l Run Time l
EOQ with Incremental Replenishment (EPQ) ¢ Definitions S = Setup Cost l H = Holding Cost l Imax = Maximum Inventory l Iavg = Average Inventory l D = Demand/Year l p = Production or Delivery Rate l u = Usage Rate l
EOQ with Incremental Replenishment Total Cost = Carrying Cost + Setup Cost Economic run quantity (Imax/2) H + (D/Qo) S Qo = 2 DS/H * p/(p-u) Cycle time (time between Qo /u runs) Run time (production Qo /p phase) Maximum Inventory Level Imax = (Qo /p)(p-u) Average Inventory Level Iaverage = Imax /2
Assumptions Only one item is involved ¢ Annual demand is known ¢ Usage rate is constant ¢ Usage occurs continually, production periodically ¢ Production rate is constant ¢ Lead time doesn’t vary ¢ No quantity discounts ¢
EOQ Replenishment Example A toy manufacturer uses 48, 000 rubber wheels per year for its product. 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 wheels is $45. The firm operates 240 days per year. Determine the: l. Optimal run size l. Minimum total annual cost for carrying and setup l. Cycle time for the optimal run size l. Run time
1 2 3 4 600 1200 1800 1600 5 6 1400 1200 7 1000 8 9 800 600 10 400 11 200 12 0 13 14 START ALL OVER 15
Other Considerations Safety Stock ¢ Reorder Point ¢ Seasonality ¢
HW #13 A mail-order house uses 18, 000 boxes a year. Carrying costs are $. 60 per box per year and ordering costs are $96. The following price schedule is offered. Determine the EOQ and the # of orders per year. # Boxes Unit Price 1000 -1999 $1. 25 2000 -4999 $1. 20 5000 -9999 $1. 15 10000+ $1. 10
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