- Classifying inventory according to some measure of importance and allocating control efforts accordingly.
- A - very important
- B - mod. important
- C - least important
Economic Order Quantity Models - Economic order quantity (EOQ) model
- The order size that minimizes total annual cost
- Economic production model
- Quantity discount 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
The Inventory Cycle - Profile of Inventory Level Over Time
Total Cost - Q is Order Quantity (in units)
- H is Holding (Carrying) cost per unit
- D is Demand, usually in units per year
- S is Ordering Cost per order
Cost Minimization Goal - The Total-Cost Curve is U-Shaped
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.
Minimum Total Cost - The total cost curve reaches its minimum where the carrying and ordering costs are equal.
EOQ Example - A local distributor for a national tire company expects to sell approximately 9,600 steel-belted radial tires of a certain size and tread design next year. Annual carrying cost is $16 per tire, and ordering cost is $75. The distributor operates 288 days a year.
- What is the EOQ?
- How many times per year does the store reorder?
- What is the length of an order cycle (time between orders)?
- What is the total annual cost if the EOQ quantity is ordered?
EOQ Example EOQ Example - Piddling Manufacturing assembles security monitors. It purchases 3,600 black-and-white cathode ray tubes a year at $65 each. Ordering costs are $31, and annual carrying costs are 20 percent of the purchase price. Compute the optimal quantity and the total annual cost of ordering and carrying the inventory.
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
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
Economic Run Size EPQ Example - A toy manufacturer uses 48,000 rubber wheels 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 wheels is $45. The firm operates 240 days per year. Determine the:
- Optimal run size
- Minimum total annual cost for carrying and setup
- Cycle time for the optimal run size
- Run time
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