Week 6 Inventory Management

1. Inventory Management

2. Definitions
 Inventory-A physical resource that a firm
holds in stock with the intent of selling it or
transforming it into a more valuable state.
 Inventory System- A set of policies and
controls that monitors levels of inventory and
determines what levels should be maintained,
when stock should be replenished, and how
large orders should be

3. Inventory
 Def. - A physical resource that a firm holds in
stock with the intent of selling it or
transforming it into a more valuable state.
 Raw Materials
 Works-in-Process
 Finished Goods
 Maintenance, Repair and Operating (MRO)

4. Expensive Stuff
 The average carrying cost of inventory
across all mfg.. in the U.S. is 30-35% of
its value.
 What does that mean?
 Savings from reduced inventory result
in increased profit.

5. Zero Inventory?
 Reducing amounts of raw materials and
purchased parts and subassemblies by
having suppliers deliver them directly.
 Reducing the amount of works-in process
by using just-in-time production.
 Reducing the amount of finished goods by
shipping to markets as soon as possible.

6. Inventory Positions in the
Supply Chain
Raw Works
Materials Finished Finished
in Goods Goods
Process in Field

7. Reasons for Inventories
 Improve customer service
 Economies of purchasing
 Economies of production
 Transportation savings
 Hedge against future
 Unplanned shocks (labor strikes, natural
disasters, surges in demand, etc.)
 To maintain independence of supply chain

8. Inventory and Value
 Remember this?
 Quality
 Speed
 Flexibility
 Cost

9. Nature of Inventory: Adding
Value through Inventory
 Quality - inventory can be a “buffer” against poor
quality; conversely, low inventory levels may force
high quality
 Speed - location of inventory has gigantic effect on
speed
 Flexibility - location, level of anticipatory inventory
both have effects
 Cost - direct: purchasing, delivery, manufacturing
indirect: holding, stockout.
HR systems may promote this-3 year postings

10. Nature of Inventory:
Functional Roles of Inventory
 Transit
 Buffer
 Seasonal
 Decoupling
 Speculative
 Lot Sizing or Cycle
 Mistakes

11. Design of Inventory Mgmt.
Systems: Macro Issues
 Need for Finished Goods Inventories
 Need to satisfy internal or external customers?
 Can someone else in the value chain carry the
inventory?
 Ownership of Inventories
 Specific Contents of Inventories
 Locations of Inventories
 Tracking

12. How to Measure Inventory
 The Dilemma: closely monitor and control
inventories to keep them as low as possible
while providing acceptable customer service.
 Average Aggregate Inventory Value:
how much of the company’s total assets
are invested in inventory?
 Ford:6.825 billion
 Sears: 4.039 billion

13. Inventory Measures
 Weeks of Supply
 Ford: 3.51 weeks
 Sears: 9.2 weeks
 Inventory Turnover (Turns)
 Ford: 14.8 turns
 Sears: 5.7 turns
 GM: 8 turns
 Toyota: 35 turns

14. Reasons Against Inventory
 Non-value added costs
 Opportunity cost
 Complacency
 Inventory deteriorates, becomes
obsolete, lost, stolen, etc.

15. Inventory Costs
 Procurement costs
 Carrying costs
 Out-of-stock costs

16. Procurement Costs
 Order processing
 Shipping
 Handling
 Purchasing cost: c(x)= $100 + $5x
 Mfg. cost: c(x)=$1,000 + $10x

17. Carrying Costs
 Capital (opportunity) costs
 Inventory risk costs
 Space costs
 Inventory service costs

18. Out-of-Stock Costs
 Lost sales cost
 Back-order cost

19. Independent Demand
 Independent demand items are
finished products or parts that are
shipped as end items to customers.
 Forecasting plays a critical role
 Due to uncertainty- extra units must
be carried in inventory

20. Dependent Demand
 Dependent demand items are raw
materials, component parts, or
subassemblies that are used to produce
a finished product.
 MRP systems---next week

21. Design of Inventory Mgmt.
Systems: Micro Issues
 Order Quantity
Economic Order Quantity
 Order Timing
 Reorder Point

22. Objectives of Inventory
Control
 1) Maximize the level of customer
service by avoiding understocking.
 2) Promote efficiency in production and
purchasing by minimizing the cost of
providing an adequate level of customer
service.

23. Balance in Inventory Levels
 When should the company replenish its
inventory, or when should the company
place an order or manufacture a new
lot?
 How much should the company order or
produce?
 Next: Economic Order Quantity

24. Models for Inventory Management:
EOQ
 EOQ minimizes the sum of holding and setup
costs
 Q = 2DCo/Ch
D = annual demand
Co = ordering/setup costs
Ch = cost of holding one unit of inventory

25. Seatide
 EOQ = 2DCo/Ch
D = annual demand = 6,000
Co = ordering/setup costs = $60
Ch = cost of holding one unit of inventory
$3.00 x 24% = .72
720,000
2 x 6,000 x 60 .72 1,000
.72

26. Marginal Analysis
Holding
Costs
$
Ordering
Costs
Units

27. Reorder Point
 Quantity to which inventory is allowed to drop
before replenishment order is made
 Need to order EOQ at the Reorder Point:
ROP = D X LT
D = Demand rate per period
LT = lead time in periods

28. Sawtooth Model
level of inventory average
inventory
units
Q
t time

29. Q - System Inventory
Control
 based on reorder point - When
inventory is depleted to ROP, order
replenishment of quantity EOQ.

30. Order Quantities
 when demand is smooth and
continuous, can operate response-
based system by determining
 best quantity to replenish periodic demand
(EOQ)
 frequency of replenishment (ROP)
 Reorder Point

31. Planning for Uncertainty
 changing lead times
 changing demand
 Uncertainty creeps in:
 Plug in safety stock
Safety stock - allows manager to
determine the probability of stock levels
- based on desired customer service
levels

32. Inventory Model Under
Uncertainty
reorder Qm
point
safety stock
time

33. Models for Inventory Management:
Quantity Discount
 Basically EOQ with quantity discounts
 To solve:
1. Write out the total cost equation
2. Solve EOQ at highest price and no discounts
3. If Qmin falls in a range with a lower price,
recalculate EOQ assuming holding cost for that
range. Call this Q2.
4. Evaluate the total cost equation at Q2 at the next
highest price break point.
OR Use a spreadsheet

34. P-System
Periodic Review Method
 an alternative to ROP/Q-system control is
periodic review method
 Q-system - each stock item reordered at
different times - complex, no economies of
scope or common prod./transport runs
 P-system - inventory levels for multiple stock
items reviewed at same time - can be
reordered together
 higher carrying costs - not optimum, but
more practical

35. Using P-System
 audit inventory level at interval (T)
 quantity to place on order is difference
between max. quantity (M) and amount
on hand at time of review
 management task - set optimal T and M
to balance stock availability and cost
 In ABC analysis, which items would use
P-system???

36. Types of Inventory Systems
 By Degree of Control required
 often use grouping method, such as ABC

37. Classifying Inventory Items
 ABC Classification (Pareto Principle)
 A Items: very tight control, complete and
accurate records, frequent review
 B Items: less tightly controlled, good
records, regular review
 C Items: simplest controls possible, minimal
records, large inventories, periodic review
and reorder

38. Does ABC Classification Make
Sense for an Assembler?
 i.e. – Gateway Computers

39. Planning Supply Chain Activities
Anticipatory - allocate supply to each
warehouse based on the forecast
Response-based - replenish inventory
with order sizes based on specific
needs of each warehouse

40. Anticipatory Inventory
Control
 determine requirements by forecasting
demand for the next production run or
purchase
 establish current on-hand quantities
 add appropriate safety stock based on
desired stock availability levels and
uncertainty demand levels
 determine how much new production or
purchase needed (total needed - on-hand)

41. Response-Based System
 replenishment, production, or purchases of
stock are made only when it has been
signaled that there is a need for product
downstream
 requires shorter order cycle time, often more
frequent, lower volume orders
 determine stock requirements to meet only
most immediate planning period (usually
about 3 weeks)

42. Service Level Achieved
•Item fill rate (IFR): the probability of filling
an order for 1 item from current stock
expected number of units out of stock/year
1-
total annual demand
•Weighted Average Fill Rate (WAFR): multiply
IFR for each stock item on an order weighted
by the ordering frequency for the item