introduction to inventory
DESCRIPTION
Introduction to Inventory. Total Cost Analysis Computation of Safety Stocks Setting Order Points Joint Replenishment Number of Facilities and Inventory. Total Cost Analysis. Start with EOQ Identify alternatives Vendor discounts Transportation options Other issues - PowerPoint PPT PresentationTRANSCRIPT
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Transportation/Logistic StrategyIntroduction to Inventory
Total Cost Analysis Computation of Safety StocksSetting Order PointsJoint ReplenishmentNumber of Facilities and Inventory
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Transportation/Logistic StrategyTotal Cost Analysis
Start with EOQIdentify alternatives
Vendor discountsTransportation optionsOther issues
Examine only total relevant costsSelect least cost alternative
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Transportation/Logistic StrategyEconomic Order Quantity
Order Quantity
Cost of Ordering
Cost of CarryingInventory
Total Cost$
*
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Transportation/Logistic Strategy
Interest and Opportunity Cost 10-20 %
Storage and Handling 8-14 %
Depreciation and Obsolescence 6-12 %
Insurance and Taxes 2 -3 %
Total 25-50 %
Elements of Inventory Carrying Costs
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Transportation/Logistic Strategy
Order Quantity
Cost of Ordering
Cost of CarryingInventory
Total Cost
$ (With Constant Ordering Costs)
Zero InventoryJIT
Economic Order Quantity
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Transportation/Logistic Strategy
Total Cost = OC + CC
OC = Order Placement Cost = A(R/Q)CC = Inventory Carrying Cost = 1/2(QVW)
Where:Q = Optimal Order Quantity (EOQ)A = Cost of placing an orderR = Annual Rate of useV = Value per unitW = Carrying cost as a percentage of average value of inventory
Determining EOQ
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Transportation/Logistic StrategyAverage Inventory
Qua
ntity
Time
AverageInventory
Inventory Carrying Cost = 1/2 (QVW)Where:
Q=Order QuantityV=Value per UnitW= Carrying Cost as a Percentage of the Average Value of Inventory
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Transportation/Logistic Strategy
Total Cost = OC + CC
OC = A(R/Q)CC = 1/2(QVW)
Total Cost = A(R/Q) + 1/2(QVW)
and solve for QTo minimize cost, set 1st derivative = zero
TC = ARQ-1 + .5QVWAR QVWQ 2+TC = or
TC’ = -ARQ-2 + .5VW or -AR VWQ2 2+TC’ = = 0
AR VWQ2 2= or 2AR
VW Q2=
Q = 2ARVW
Determining EOQ
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Transportation/Logistic Strategy
Total Cost = OC + CC + Tr + PC + It + SS + Other
Where:OC = Order Placement CostCC = Inventory Carrying CostTr = Transportation CostPC = Product CostIt = Inventory in Transit CostSS = Safety Stock Cost
Total Cost Analysis
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Transportation/Logistic Strategy
Total Cost = OC + CC + Tr + PC + It + SS + OtherOC = A(R/Q)CC = 1/2(QVW)Tr = rRwt/100PC = VRIt = iVRt/365SS = BVWWhere:
Q, R, A, V, W = As previously definedr = Transportation rate per 100 pounds (CWT)wt = Weight per uniti = Interest rate or cost of capitalt = Lead time in daysB = Buffer of inventory to prevent stockouts
Total Cost Analysis
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Transportation/Logistic StrategyTotal Cost without Coordination of Purchasing and Transportation
TC
CC
PC
TROC
Quantity
~
~
$
r1 r2 p2p1
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Transportation/Logistic StrategyTotal Cost without Coordination of Purchasing and Transportation
PC
TROC
CC
TC$
Quantity
~
~
r1/p1 r2/p2
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Transportation/Logistic Strategy
If sales or lead times vary, safety stock is necessary to maintain desired service levels.
Computing Safety Stock
Begin with Order Point
(BVW)
If sales are constant and there is no variation in delivery times,no buffer is necessary, i.e., safety stock is not a relevant cost.
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Transportation/Logistic StrategySetting Order Points
Order Point = Daily Use X Lead Time in Days
Place orders when stock level falls to the level of the order point.
Lead Time
Qua
n tity
OrderPoint
Time
No Uncertainty
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Transportation/Logistic Strategy
Order Point = Mean Daily Use X Lead Time in DaysTo Protect Against Stockouts, Place Orders SoonerBut How Much Sooner?
Lead Time
Qu a
n tity
OrderPoint
TimeStockout
Setting Order PointsWith Uncertainty
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Transportation/Logistic Strategy
Place orders when stock levels fall to the mean, and half the time you have enough to cover sales and half the time you don’t; i.e., you have a 50% Fill Rate.
Raise the order point, and you raise the service level.
One Standard Deviation of Buffer Stock Provides an 84% Fill Rate
Two Standard deviations of Buffer Stock Provides a 97.5% Fill Rate
How Do We Know?
Example
Setting Order PointsWith Uncertainty
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Transportation/Logistic StrategyAverage Daily Sales
SalesDay (Xi) (Xi ‑ X) (Xi – X)2
1 100 0 02 90 ‑10 100 3 110 10 100 4 85 15 225 5 105 5 25 6 110 10 100 7 95 ‑5 25 8 115 15 225 9 100 0 0 10 90 ‑10 100 11 110 10 100 12 90 ‑10 100 SUM 1200 0 1100Mean = 100; Std Dev = 10
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Transportation/Logistic StrategyMean and Standard Deviation
of Average Daily Sales
Mean = 1200/12 = 100 ___________ ________
Std Dev = √ (Xi – X)2 = √1100/11 = 10 n-1
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Transportation/Logistic Strategy
Assume constant delivery time of one day, i.e., no variation
Order Point = Mean Daily Use X Lead Time in Days + Buffer Stock
Mean Sales = 100 units
Standard Deviation of Sales = 10 units
Order Point = 100 + 10 = 110
Setting Order PointsWith Uncertainty
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Transportation/Logistic Strategy
Order Point = Daily Use X Lead Time in Days + Buffer Stock
By Ordering Sooner, Stockouts are Avoided
Lead Time
Qua
ntity
110
Time
100
What is the service level (fill rate) associated with this order point?
Setting Order PointsWith Uncertainty
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Transportation/Logistic Strategy
One Standard Deviation = 68% of the area.
Half of the time sales are below 100; 84% of the time sales are below 110.
68%
34% 34%
10090Sales 110
Computing Safety Stock
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Transportation/Logistic Strategy
Two Standard Deviations = 95% of the area.
Half the time sales are below 100; 97.5% of the time sales are below 120.
95%
47.5% 47.5%
10080Sales 120
Computing Safety Stock
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Transportation/Logistic Strategy z 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.0 0.0000 0.0040 0.0080 0.0120 0.0160 0.0199 0.0239 0.0279 0.0319 0.03590.1 0.3980 0.0438 0.0478 0.0517 0.0557 0.0596 0.0636 0.0675 0.0714 0.07530.2 0.0793 0.0832 0.0871 0.0910 0.0948 0.0987 0.1026 0.1064 0.1103 0.11410.3 0.1179 0.1217 0.1255 0.1293 0.1331 0.1368 0.1406 0.1443 0.1480 0.15170.4 0.1554 0.1591 0.1628 0.1664 0.1700 0.1736 0.1772 0.1808 0.1844 0.18790.5 0.1915 0.1950 0.1985 0.2019 0.2054 0.2088 0.2123 0.2157 0.2190 0.22240.6 0.2257 0.2291 0.2324 0.2357 0.2389 0.2422 0.2454 0.2486 0.2518 0.25490.7 0.2580 0.2612 0.2642 0.2673 0.2704 0.2734 0.2764 0.2794 0.2823 0.28520.8 0.2881 0.2910 0.2939 0.2967 0.2995 0.3023 0.3051 0.3078 0.3106 0.31330.9 0.3159 0.3186 0.3212 0.3238 0.3264 0.3289 0.3315 0.3340 0.3365 0.33891.0 0.3413 0.3438 0.3461 0.3485 0.3508 0.3531 0.3554 0.3577 0.3599 0.36211.1 0.3643 0.3665 0.3686 0.3708 0.3729 0.3749 0.3770 0.3790 0.3810 0.38301.2 0.3849 0.3869 0.3888 0.3907 0.3925 0.3944 0.3962 0.3980 0.3997 0.40151.3 0.4032 0.4049 0.4066 0.4082 0.4099 0.4115 0.4131 0.4147 0.4162 0.41771.4 0.4192 0.4207 0.4222 0.4236 0.4251 0.4265 0.4279 0.4292 0.4306 0.43191.5 0.4332 0.4345 0.4357 0.4370 0.4382 0.4394 0.4406 0.4418 0.4429 0.44411.6 0.4452 0.4463 0.4474 0.4484 0.4495 0.4505 0.4515 0.4525 0.4535 0.45451.7 0.4554 0.4564 0.4573 0.4582 0.4591 0.4599 0.4608 0.4616 0.4625 0.46331.8 0.4641 0.4649 0.4656 0.4664 0.4610 0.4678 0.4686 0.4693 0.4699 0.47061.9 0.4713 0.4719 0.4726 0.4732 0.4738 0.4744 0.4750 0.4756 0.4761 0.47672.0 0.4772 0.4778 0.4783 0.4788 0.4793 0.4798 0.4803 0.4808 0.4812 0.48172.1 0.4822 0.4826 0.4830 0.4834 0.4838 0.4842 0.4846 0.4850 0.4854 0.48572.2 0.4861 0.4864 0.4868 0.4871 0.4875 0.4878 0.4881 0.4884 0.4887 0.48902.3 0.4893 0.4896 0.4898 0.4901 0.4904 0.4906 0.4909 0.4911 0.4911 0.49162.4 0.4918 0.4920 0.4922 0.4925 0.4927 0.4929 0.4931 0.4932 0.4934 0.49362.5 0.4938 0.4940 0.4941 0.4943 0.4945 0.4946 0.4948 0.4949 0.4951 0.49522.6 0.4953 0.4955 0.4956 0.4957 0.4959 0.4960 0.4961 0.4962 0.4963 0.49642.7 0.4965 0.4966 0.4967 0.4968 0.4969 0.4900 0.4971 0.4972 0.4973 0.49742.8 0.4974 0.4975 0.4976 0.4977 0.4977 0.4978 0.4979 0.4979 0.4980 0.49812.9 0.4981 0.4982 0.4982 0.4983 0.4984 0.4984 0.4985 0.4985 0.4986 0.49863.0 0.4986 0.4987 0.4987 0.4988 0.4988 0.4989 0.4989 0.4989 0.4990 0.49904.0 0.49997
\\\\\\\
Areas under the standard normal probability distribution between the mean and successive values of z
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Transportation/Logistic StrategyEffects of Delivery Time Variation
Where:SDt = Standard Deviation of Demand over time(Units
of Safety Stock required to satisfy 68 percent of sales levels during lead time)
t = Average delivery timeSt = Standard Deviation of delivery timeD = Average DemandSD = Standard Deviation of Demand
SDt = (t)(SD)2 + (D)2 (St)2
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Transportation/Logistic StrategyExample
ASSUMPTIONS Average Sales (D) = 100 unitsStandard Deviation of Sales (SD ) = 10 units
CARRIER A CARRIER B Average Transit Time (t) 5 Days 5 Days Standard Deviation of Time (St) 1 Day 2 Days
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Transportation/Logistic Strategy
Combined standard deviations
CARRIER A
SDt = (5) (10)2 + (100)2 (1)2 = 102 units
SDt = √ (5) (10)2 + (100)2 (2)2 = 201 units
CARRIER B
(Continued)
Example
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Transportation/Logistic Strategy
Assumptions:
Cost of Carrying Safety Stock = BVW
(Buffer Stock) X (Value) X (Carrying Cost)
Value per Unit = $100.00
Carrying Cost = 20 percent
Service Level = 97.5 percent a ________________________________________________
a To attain a 97.5% service level, the shipper will have to
add 2 standard deviations of safety stock to the order point.
(Continued)
Example
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Transportation/Logistic Strategy
CARRIER A: SDt = 102 units
Cost = 2 X 102 X 100 X .2 = $4,080
CARRIER B: SDt = 201 units
Cost = 2 X 201 X 100 X .2 = $8,040
Additional cost of using a less reliable carrier:
($8,040 ‑ $4,080) = $3,960
(Continued)
Example
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Transportation/Logistic Strategy
Basic Trade-offMore facilities = more inventoryMore facilities = shorter shipments
Tie in with ABC AnalysisConsolidate slow moving items in central locationsTrade increased transportation for reduced inventory
or vice versa
Inventory and the Number of Facilities
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Transportation/Logistic StrategyInventory and the Number of Facilities
Number of Facilities (X)
$(Y)
Y= b X
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Transportation/Logistic StrategyJoint Replenishment
Order multiple items at the same time.
When any item hits an order point, place orders for all items.
Constraint: Order only full truckloads (e.g., 40,000 lbs).
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Transportation/Logistic Strategy
Qi = RiQ + I - P
R- Ii + Pi( )
Where:Qi = Order Quantity of the ith item (to be determined)Ri = Annual Rate of Use of the ith itemIi = Inventory on hand for the ith itemPi = Reorder point for the ith item
Q = Total Shipment SizeR = Total of Individual Usage RateI = Total of Individual Inventory Items on HandP = Total of Individual Reorder Points
Joint Replenishment
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Transportation/Logistic Strategy
Example: Corn, Beans, PeasUnits
Ri Pi Ii
Corn 6,400 163 154Beans 9,800 218 250Peas 5,100 150 180 Totals 21,300 531 584
Cases =25 pounds Ri Pi Ii
Corn 160,000 4,075 3,850Beans 245,000 5,450 6,250Peas 127,000 3,750 4,500 Totals 532,500 13,275 14,600
R P I
Joint Replenishment
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Transportation/Logistic Strategy
Qi = RiQ + I - P
R- Ii + Pi( )
Corn:Qi = 160,000 (40,000+14,600-13,275) - 3,850 + 4,075 = 12,641
532,500 Beans:Qi = 245,000(40,000+14,600-13,275) - 6,250 + 5,450 = 18,213
532,500Peas:Qi = 127,500(40,000+14,600-13,275) - 4,500 + 3,750 = 9,145
532,500 40,000
Example: Corn, Beans, PeasJoint Replenishment