INVENTORY MANAGEMENT
Inventory Management
Importance of Inventory Management -- Good inventory management is essential to the successful operation for most organizations because of:
1. The amount of money invested in inventory represents, and
2. The impact that inventories have on daily operations of an organization
Definitions:
Inventory – a stock or store of goods
Independent vs. Dependent demand items
Independent demand items are the finished goods or other end items that are sold to someone
Dependent demand items are typically subassemblies or component parts that will be used in the production of a final or finished product
Our focus: inventory management of finished goods, raw materials, purchased parts, and retail items
Functions of Inventories
1. To meet anticipated demand
2. To smooth production requirements
3. To decouple components of the production
4. To protect against stockouts
5. To take advantage of order cycles
6. To hedge against price increases, or to take advantage of quantity discounts
7. To permit operations (work in process)
Objectives of Inventory Control
1. Maximize level of customer service
2. Minimize costs (carrying costs and ordering costs)
Requirements for Effective Inventory Management
(1) A system to keep track of the inventory
▪ periodic,
▪ perpetual,
▪ two-bin, and
▪ universal product code (UPC)
(2) A reliable forecast of demand
(3) Knowledge of lead times and lead time variability
-lead time ( time between submitting a purchase order and receiving it
-lead time variability ( reliability of the supplier
(4) Estimates of inventory holding costs, ordering costs, and shortage costs
Holding cost
Ordering cost
Stockout cost
(5) A classification system for inventory items
ABC approach – classifies inventory
according to some measure of importance
($ value) where A – very important,
C – least important
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Formula for EOQ with Non-instantaneous Replenishment
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where: D – annual demand
S – setup cost
H – Holding (carrying cost) per unit
p – production or delivery rate
d – usage rate
C. Quantity Discounts Model
1. Compute the common EOQ
2. Only one of the unit prices will have the EOQ in its feasible range. Identify the range that:
• If the feasible EOQ is on the lowest price range, that is the optimal order quantity
• If the feasible EOQ is in any other range, compute the total cost for the EOQ and for the price breaks of all lower unit costs. Compare the total costs – EOQ is the one that yields the lowest total cost.
When to Order (reorder points - ROPs) Models
Objective: minimize the risk (probability) of stockouts
4 Determinants of the ROP
1. rate of demand
2. lead time
3. extent of demand and/or lead time variability
4. degree of stockout risk acceptable to management
Basic Formula for Computing ROP
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A. Constant demand and constant lead time
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B. Variability is present in demand during lead time
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use this formula if an estimate of expected demand during lead time and its standard deviation are available
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use this formula when data on lead time and demand are not readily available
Shortages and Service Levels
The ROP computation does not reveal the expected amount of shortage for a given lead time service level
Information on expected number of shortage per cycle, or per year can be determined using the following:
A. Expected number of units short per cycle, E(n)
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B. Expected number of units short per year, E(N)
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C. Annual Service Level
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Service Level for Single-period Model
Used to handle ordering of perishables
(fresh fruits, vegetables, seafood, flowers), and
Items that have a limited useful life
(newspaper, magazines)
Analysis focuses on two costs: shortage and excess
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Problems:
2 – ABC Inventory Classification
3 – Basic EOQ
4 – Basic EOQ
11– EOQ with Non-instantaneous Delivery
13 – EOQ with Discount
28 – EOQ, ROP, Shortages
33 – EOQ for multiple products
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(b) Determine the EOQ for each item.
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