copyright ©2003 stanley b. gershwin....3 copyright © 2000 lawrence m. wein. newsboy model •...
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Copyright ©2003 Stanley B. Gershwin.
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Inventory Theory Inventory Theory
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3 Motives for Holding Inventory
1) seasonal inventory: due to seasonality (predictable)
2) buffer inventory (= safety stock): due to random fluctuations (unpredictable)
3) cycle inventory: due to economies of scale
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Newsboy Model• Buying a perishable item• Given forecast of future sales• Excess demand is lost• Excess supply is salvaged
c = purchase cost per unitr = revenue per units = salvage value per unitd = unknown demand
marginal cost = cmarginal revenue = r if d ≥Q
s if d <Q
MC = E MR[ ]c = r Pr(d ≥Q ) +sPr(d <Q )
Order Q, where Pr(d ≤Q ) = r -cr -s
f (d )
Q
r - cr - s
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Copyright ©2003 Stanley B. Gershwin.
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Copyright ©2003 Stanley B. Gershwin.
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Copyright ©2003 Stanley B. Gershwin.
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Copyright ©2003 Stanley B. Gershwin.
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Copyright ©2003 Stanley B. Gershwin.
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Copyright ©2003 Stanley B. Gershwin.
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Copyright ©2003 Stanley B. Gershwin.
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Copyright ©2003 Stanley B. Gershwin.
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Copyright ©2003 Stanley B. Gershwin.
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DDI ExampleDDI Exampled = Demand ~Normal µ = 150,000 σ = 45,000r = $150c = $50s = $0
Pr(d ≤Q ) =r −cr − s
= .667
Pr(d ≤Q ) = Pr d − 150,00045,000
≤Q − 150,000
45,000⎛
⎝ ⎜
⎞
⎠ ⎟
= Pr Z ≤Q − 150,000
45,000⎛
⎝ ⎜
⎞
⎠ ⎟ = 0.667
0
0 .667
0 .431
Pr Z ≤ 0.431( ) = 0.667
Q − 150,00045,000
= 0.431
Q = 150,000 + 0.431 45,000( ) = 169,395
safety stock
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EOQ Modeltradeoff of holding cost vs. ordering cost
λ = annual demand rate c = variable cost per unitk = inventory carrying cost per year (%)
ck = holding cost per unit per years = fixed ordering cost
Q = order quantity (decision)
Assume1) No statistical uncertainty2) instantaneous replenishment3) no shortages allowed
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EOQ Derivation
0
Q
−λ
Qλ
2Qλ
3Qλ time
inventory
cost / year = sλQ +ckQ 2 +cλ
d (cos t )dQ = 0 ⇒Q * = 2λs
ck = EOQ
1) EOQ is robust Cost Q *2( )= cost 2Q *( ) = 1.25 cost (Q *)
2) s ↑ ⇒ Q * ↑
3) s ↓ ⇒ Q * ↓
4) λ ↑ ⇒ Q* ↑
5) c ↑ ⇒ Q * ↓
Comments
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DellpaqDellpaq ExampleExample λ = 300,000 per year s = $100,000 + $50 = $100,050 c = $3,000 + $25 + $1+ $5 + $0.50 = $3,031.50 k = 0.20Q* = 2sλ
ck = 9950.4
What is re-order point?
Lead time = 1 week9950.4 units
300,000 units/year⎛
⎝ ⎜
⎞
⎠ ⎟
52 weeksyear
= 1.72 weeks
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DellpaqDellpaq Example Cont.Example Cont.Lead time = 1 week
9950.4 units300,000 units/year
⎛
⎝ ⎜
⎞
⎠ ⎟
52 weeksyear
= 1.72 weeks
1 week
9950
5769
-300K/yr
1.72 weeks Reorder Point = inventory level at time of ordering = demand during lead time
= 300,000 units/year52 weeks/year
⎛
⎝ ⎜
⎞
⎠ ⎟ 1 week = 5769 units
Reorder Point Policy = when inventory level drops to 5769 units, order 9950 units
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Inventory PositionInventory PositionQ: What if lead time = 3 weeks?
• Then you make an order before the previous order has arrived
• Inventory position = inventory on hand + inventory on order
• You track the inventory position, and use a reorder point policy with respect to the inventory position
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Inventory PositionInventory Position
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Key insight: the current order must satisfy demanduntil the next order arrives
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(R,Q)(R,Q) PolicyPolicyQ: What if demand is uncertain?
(R,Q) policy: when inventory position drops to R, order Q
Simple Heuristic: Set Q = EOQLet = DDLT = demand during the lead timeWe need R units to satisfy DDLTUse newsboy modelSet R so that Pr DDLT ≤R( ) =
r −cr −s
Image removed due to copyright considerations.
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ExampleExampleL = deterministic lead timeDi = iid demand in each period with mean E D[ ] and variance σ2 D[ ]
DDLT =D1 + ... +DL
E DDLT[ ]= LE D[ ]Var DDLT[ ]= Lσ2 D[ ]
set R so that Pr DDLT ≤R( )
= Pr DDLT −LE D[ ]Lσ D[ ]
≤R −LE D[ ]Lσ D[ ]
⎛
⎝ ⎜
⎞
⎠ ⎟
= r −cr −s