Upload
arron-morrison
View
218
Download
0
Tags:
Embed Size (px)
Citation preview
11
Forecastingand
Logistics
John H. Vande Vate
Fall, 2002
22
Basics
• Read the text for forecasting basics
• Will not spend class time on the mechanics
33
Fundamental Rules
• Rule #1: The farther in the future we must forecast, the worse the forecast
• The longer we have available to do something the cheaper it is to do it.
• Balance these two– Long plans mean bad forecasts– Short plans mean high operational costs
44
Balancing Risk
• News vendor problem• A single shot at a fashion market• Guess how much to order
– If you order too much, you can only salvage the excess (perhaps at a loss) (s-c = net salvage value)
– If you order too little, you lose the opportunity to sell (r-c = profit)
• Question: What value do you choose?
55
The Idea
• Balance the risks
• Look at the last item– What did it promise?– What risk did it pose?
• If Promise is greater than the risk?
• If the Risk is greater than the promise?
66
Measuring Risk and Return • Profit from the last item
$profit if demand is greater, $0 otherwise
• Expected Profit$profit*Probability demand is greater than our choice
• Risk posed by last item$risk if demand is smaller, $0 otherwise
• Expected Risk$risk*Probability demand is smaller than our choice
Example: risk = Salvage Value - CostWhat if Salvage Value > Cost?
77
Balancing Risk and Reward• Expected Profit
$profit*Probability demand is greater than our choice
• Expected Risk$risk*Probability demand is smaller than
our choice
• How are probabilities Related?
88
Risk & RewardDistribution
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 2 4 6 8 10 12
Prob. Outcome is smaller
Prob. Outcome is larger
Our choice
How are they related?
99
Balance
• Expected Revenue$profit*(1- Probability demand is smaller than our
choice)
• Expected Risk$risk*Probability demand is smaller than our choice
• Set these equalprofit*(1-P) = -risk*Pprofit = (profit-risk)*Pprofit/(profit - risk) = P = Probability demand is smaller
than our choice
1010
Distribution
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 2 4 6 8 10 12
Making the Choice
Prob. Demand is smaller
Our choice
profit/(profit - risk)
Cumulative Probability
1111
Example
• What we sell in the month, we earn $1 per unit on
• If we hold a unit in inventory past the end of the month, we lose $0.50 because of price falls and inventory costs
• Demand forecasted as N(, )
• measures our uncertainty
1212
What to do?
• How much to ship• Last item
– If we sell it• Earn $1 with probability that demand exceeds amount
• (1-P)
– If we fail to sell it• Pay $0.50 with probability that demand falls short
• -0.5P
• So, we want P to be 1/(1+.5) = 2/3 ~ .67• Go look that up in the N(, )
1313
Some Intuition
• Profit = $1, Risk = -$1
• Mean = 1000, Std Dev = 100
• What’s the best strategy?
-0.0005
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
0.004
0.0045
0 200 400 600 800 1000 1200 1400
Order the average. Return~$920
Why less than $1,000?
1414
What happens?
• What happens to return as – Increases?– Decreases?
• What happens to as lead time– Increases?– Decreases?
• What happens to return as lead time – Increases? – Decreases?
1515
Extend Idea
• Ship too little, you have to expedite the rest• Expedite Cost• Ship Q• If demand < Q
– We sell demand and salvage (Q – demand)
• If demand > Q– We sell demand and expedite (demand – Q)
• What’s the strategy?
1616
Same idea
• Ignore profit from sales – that’s independent of Q
• Focus on salvage and expedite costs• Look at last item
– Chance we salvage it is P– Chance we expedite it is (1-P)
• Balance these costs– Unit salvage cost * P = Unit expedite cost (1-P)– P = expedite/(expedite + salvage)
1717
Another View
• Rule #2: The less detailed the subject matter, the more accurate the forecast
1818
Safety Stock
• Protection against variability– Variability in lead time and
– Variability in demand, etc.
– Typically described as days of supply
– Should be described as standard deviations in lead time demand
– Example: BMW safety stock • For axles only protects against lead time variability
• For option parts protects against usage variability too
1919
Traditional Basics
• Basic tool to manage risk
Time
Sto
ck o
n ha
nd
Safety Stock
Reorder Point
Order placed
Lead Time
Actual Lead Time Demand
Avg LT Demand
2020
Safety Stock Basics
• n customers
• Each with lead time demand N(, )
• Individual safety stock levels– Choose z from N(0,1) to get correct
probability that lead time demand exceeds z,– Safety stock for each customer is z– Total safety stock is nz
2121
Safety Stock Basics
• Collective Lead time demand N(n, n)• This is true if their demands and leadtimes are
independent!• Collective safety stock is nz• Typically demands are negatively or positively
correlated• What happens to the collective safety stock if
demands are – positively correlated?– Negatively correlated?
2222
Inventory (Risk) PoolingHistorical Data for Product A
1 2 3 4 5 6 7 8Massachusetts 33 45 37 38 55 30 18 58New Jersey 46 35 41 40 26 48 18 55Pooled 79 80 78 78 81 78 36 113
Average Std DevCoeff of
Var
Avg. Lead time
DemandSafety Stock
Reorder Point EOQ
Order Up To Level
Avg. Inventory
Massachusetts 39.25 13.18 0.34 39.25 24.78 64 132 196 91New Jersey 38.63 12.05 0.31 38.63 22.66 61 131 192 88Pooled 77.88 20.71 0.27 77.88 38.95 117 186 303 132
Historical Data for Product B1 2 3 4 5 6 7 8
Massachusetts 0 2 3 0 0 1 3 0New Jersey 2 4 0 0 3 1 0 0Pooled 2 6 3 0 3 2 3 0
Average Std DevCoeff of
Var
Avg. Lead time
DemandSafety Stock
Reorder Point EOQ
Order Up To Level
Avg. Inventory
Massachusetts 1.13 1.36 1.21 1.13 2.55 4 22 26 14New Jersey 1.25 1.58 1.26 1.25 2.97 4 24 28 15Pooled 2.38 1.92 0.81 2.38 3.62 6 32 38 20
Inventory ComparisonMassachusetts New Jersey Total Pooled Reduction
Product A 91 88 179 132 26%Product B 14 15 28 20 30%Total 105 103 207 152 27%
Week
Week
Pooling Inventory can reduce safety stock
The impact is less than the sqrt of 2 law
It predicts that if 2 DCs need 47 units then a single DC will need
33
The impact is greater than the sqrt of 2 law
It predicts that if 2 DCs need 5.5 units
then a single DC will need 4
2323
Inventory (Risk) Pooling
• Centralizing inventory can reduce safety stock
• Best results with high variability and uncorrelated or negatively correlated demands
• Postponement ~ risk pooling across products
2424
Forecasting So What
• Mechanics of forecasting– Review the past
– Project it into the future
• What to do with forecasts?– Build a business case with the means (planning)
– Assess risks with the std deviations (hedging)
• Real question is– Not how to forecast better, but
– How to manage risk better
2525
Examples
• Inventory Strategy– What inventories (risks) can you pool
• Supplying international operations– How much to ship– How much to expedite – How much inventory to hold– How to manage the process
• International Sourcing– What products/volumes to source from fast, expensive
local sources– What products/volumes to source from slow, long lead
time distant sources
2626
Examples cont’d
• Purchasing Strategy– What to purchase on the “spot market” – What prices to fix with contracts
• Manufacturing Strategy– What products/volumes to build-to-order– What products/volumes to build-to-stock
• Our focus on supplying international operations
2727
Supplying International Ops
• Several interwoven issues– Assessing the risk– Reducing the risk through product/supply
chain design– Managing the risks through effective supply
process
2828
Reducing the Risks
• Focus on postponement
• Postponement: Delaying the point of product differentiation
2929