Managing Service Inventory NKFUST Reasons to Hold Inventory - - PDF document

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Managing Service Inventory

• Reasons to Hold Inventory
• Inventory Models
• ABC Classification
• News Vendor Problem

Shin‐Ming Guo NKFUST

Hospitals and Inventory Management

Control

• Barcodes and computers keep track of

every bottle of antibiotics and other supplies.

• Secure supply cabinets with thumbprint

security technology

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Management

 Analyze how much is spent on every type of illness and

surgical procedure.

 Computers keep track of stock and automatically reorder

from suppliers

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Focus: Matching Supply with Demand

• Demand can vary and is unpredictable.
• Supply is inflexible and maybe costly.
• Demand < Supply  Stock may be perishable
• Demand > Supply  Customers may not want

to wait

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Who Cares About Inventory?

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Acer reported its first‐ever quarterly loss yesterday. The Taiwanese PC maker suffered a net loss of 6.79 billion in Taiwanese dollars ($234.3 million) Acer has been hit hard by

• verestimating demand for its PCs. It

has lost $150 million to get rid of excess inventory.

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What About a Shortage of Vaccine?

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In 2006, Nintendo launched the Wii game console and could not make enough units to keep up with the demand. Some people would wait in long lines to purchase scarce units and resell them

• nline for several hundred dollars over the retail price

Physical Goods Distribution

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Reasons to Hold Inventory

• In‐transit Inventory
• Seasonal Inventory
• Cycle Inventory
• Decoupling Inventory/Buffers
• Safety Inventory
• Speculative Inventory

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Reasons to Hold Less Inventory

• Inventory might become obsolete.
• Inventory might perish.
• Inventory might disappear.
• Inventory requires storage space

and other overhead cost.

• Opportunity cost.

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Inventory Costs

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 Holding or Carrying cost

Overestimate the demand

storage cost: facility, handling risk cost: depreciation, pilferage, insurance

• pportunity cost

 Ordering cost

cost placing an order: preparing, negotiating, receiving and inspection

 Shortage costs or Lost Sales

Underestimate the demand

costs of canceling an order or penalty

Annual cost ≈ 20% to 40% of the inventory’s worth

Inventory Performance

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 Inventory turn =  Service level = in‐stock probability before the

replenishment order arrives

 Fill rate =

Cost of Goods Sold ___________________ average inventory value number of demands number of sales _________________ How to calculate average Inventory value?

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Inventory Control Decisions

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Fixed Order Period P model Fixed Order Quantity Q model Ordering of general merchandise, supplies When to order? Reorder point, order frequency How many to order? Order quantity, target inventory level

Fixed Order Quantity Models

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Economic Order Quantity

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D=annual demand, Q=order quantity, S=setup cost, H=unit holding cost Total annual cost =ordering cost + holding cost Economic Order Quantity Q H DS Q 2 *  dL L d=daily demand L=lead time Reorder point =dL

H 2 Q S Q D TC(Q)  

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Inventory Control under Demand Uncertainty

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Place a new order whenever the inventory position drops to ROP (reorder point).

ROP

d=daily demand L=lead time Reorder point =dL + safety stock How to avoid possible stock out?

 amount of inventory carried in addition to the expected

demand, in order to avoid shortages when demand increases

Safety Stock

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safety stock Service level=probability of no shortage =P (demand ≤ inventory) =P(demand ≤ E(D)+safety stock)

 depends on service level, demand variability, order lead time  service level depends on Holding cost  Shortage cost

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=daily demand =std dev. of daily demand

         L z L L ROP stock safety during demand expected

Service level or probability of no shortage =95% (99%)  z=1.64 (2.33)

P model: fixed time period

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RP is the review period. TIL is the target inventory level determined by the forecasts. We place an order to bring the inventory level up to TIL.

RP RP RP

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Timespan = length of review period + lead time = RP+ L Order Quantity = target inventory – inventory position

        L RP z L RP ) (

Target Inventory = expected demand + safety stock

ABC Classification

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dollar usage=usage × cost

There are other ways to do ABC classification. Review ABC classification periodically.

Pareto’s 80/20 principle

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ABC Classification for Inventory Control

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A  Q model B  P model with R=1 week C  P model with R=1 month

Single Period Inventory Model

• Only one production or procurement opportunity.
• Stochastic demand leads to lost sales or leftover.
• There are losses of profit and goodwill for each

unsatisfied customer.

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 There is a salvage value for each

unit of leftover.

 Forecasting helps balancing cost

• f ordering too much vs. cost of
• rdering too little.

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Case : Order Management at Sport Obermeyer

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Klaus Obermeyer founded Obermeyer in 1947, when he was among the first ski instructors on Aspen Mountain.

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Customer service, marketing, design & research, accounting in Colorado Rockies.

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Contract manufacturers in Hong Kong and China.

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Long lead time, short sales period

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Increasing product variety, more marked downs

Forecasting and Ordering at Sport Obermeyer

• Demands depend on weather, fashion trend, economy.
• Forecasts based on Panel Consensus.
• Dominant members have stronger influence on the
• utcome of a consensus forecast.
• Independent forecasts can provide an indicator of the

forecast accuracy for each style.

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Working with Customers to Improve Forecasts

• Obermeyer invites key customers to place early orders (20%
• f total sales) to get market information.
• Forecasts are updated based on those early orders.

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News Vendor Problem

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D = boxes of donuts demanded Q = boxes of donuts stocked P = price of one box of donuts, $10 C = cost of one box of donuts, $4 S = salvage value of one box, $2

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P = price of one box, $10 C = cost of one box, $4 S = salvage value of one box, $2 Cu = unit contribution: P‐C = $6 Co = unit loss: C‐S = $2

Finding Optimal Order Quantity

• F(Q) = probability of having leftover inventory
• To maximize expected profit, we order Q units so that the

expected loss on the Qth unit equals the expected gain on the Qth unit:

• Rearrange the above equation ‐>
• Cu / (Co+Cu) is called the critical ratio.
• Choose Q such that the probability of no lost sales (i.e.,

demand < Q) equals the critical ratio.

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   

Q F C Q F C

u

• 

   1 ) (

u

• u

C C C Q F   ) (

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Retail Discounting Model

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 S = current selling price  D = discount price  P = profit margin on cost (% markup as decimal)  Y = average number of years to sell entire stock of “dogs” at

current price (total years to clear stock divided by 2)

 N = inventory turns (number of times stock turns in one year)

Loss per item = Gain from revenue S – D = D(PNY)

) 1 ( PNY S D   