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Introduction to Inventory

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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

Examine only total relevant costs

Select least cost alternative

*

Economic Order Quantity

$

Total Cost

Cost of Carrying

Inventory

Cost of Ordering

Order Quantity

Elements of Inventory Carrying Costs

10-20 %

Interest and Opportunity Cost

Storage and Handling

8-14 %

Depreciation and Obsolescence

6-12 %

Insurance and Taxes

2 -3 %

Total 25-50 %

Economic Order Quantity

$

(With Constant Ordering Costs)

Total Cost

Cost of Carrying

Inventory

Cost of Ordering

Zero Inventory

JIT

Order Quantity

Determining EOQ

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 order

R = Annual Rate of use

V = Value per unit

W = Carrying cost as a percentage of average value of inventory

Quantity

Average

Inventory

Time

Average Inventory

Inventory Carrying Cost = 1/2 (QVW)

Where:

Q=Order Quantity

V=Value per Unit

W= Carrying Cost as a Percentage of the Average Value of Inventory

AR

QVW

TC =

+

2

Q

or

TC = ARQ-1 + .5QVW

To minimize cost, set 1st derivative = zero

and solve for Q

-AR

VW

TC’ = -ARQ-2 + .5VW

TC’ =

= 0

or

+

2

Q2

2AR

2AR

or

Q2

=

AR

VW

VW

VW

=

2

Q2

Determining EOQ

Total Cost = OC + CC

OC = A(R/Q)

CC = 1/2(QVW)

Total Cost = A(R/Q) + 1/2(QVW)

Q=

Total Cost Analysis

Total Cost = OC + CC + Tr + PC + It + SS + Other

Where:

OC = Order Placement Cost

CC = Inventory Carrying Cost

Tr = Transportation Cost

PC = Product Cost

It = Inventory in Transit Cost

SS = Safety Stock Cost

Total Cost Analysis

Total Cost = OC + CC + Tr + PC + It + SS + Other

OC = A(R/Q)

CC = 1/2(QVW)

Tr = rRwt/100

PC = VR

It = iVRt/365

SS = BVW

Where:

Q, R, A, V, W = As previously defined

r = Transportation rate per 100 pounds (CWT)

wt = Weight per unit

i = Interest rate or cost of capital

t = Lead time in days

B = Buffer of inventory to prevent stockouts

$

TC

~

CC

~

PC

TR

OC

r1

p1

r2

p2

Quantity

Total Cost without Coordination

of Purchasing and Transportation

$

TC

~

CC

~

PC

TR

OC

r1/p1

r2/p2

Quantity

Total Cost without Coordination

of Purchasing and Transportation

Computing Safety Stock

(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.

If sales or lead times vary, safety stock is necessary to maintain

desired service levels.

Begin with Order Point

Quantity

Order

Point

Time

Lead Time

Setting Order Points

No Uncertainty

Order Point = Daily Use X Lead Time in Days

Place orders when stock level falls to the level of the

order point.

Quantity

Order

Point

Lead Time

Time

Stockout

Setting Order Points

With Uncertainty

Order Point = Mean Daily Use X Lead Time in Days

To Protect Against Stockouts, Place Orders Sooner

But How Much Sooner?

Setting Order Points

With Uncertainty

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

Sales

Day (Xi) (Xi ‑ X) (Xi – X)2

1 100 0 0

2 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 1100

Mean = 100; Std Dev = 10

Average Daily Sales

Mean and Standard Deviation

of Average Daily Sales

Mean = 1200/12 = 100

___________ ________

Std Dev =√ (Xi – X)2 = √1100/11 = 10

n-1

Assume constant delivery time of one day, i.e., no variation

Mean Sales = 100 units

Standard Deviation of Sales = 10 units

Order Point = Mean Daily Use X Lead Time in Days + Buffer Stock

Order Point = 100 + 10 = 110

Setting Order Points

With Uncertainty

Setting Order Points

With Uncertainty

Quantity

110

100

Time

Lead Time

Order Point = Daily Use X Lead Time in Days + Buffer Stock

By Ordering Sooner, Stockouts are Avoided

What is the service level (fill rate) associated with this order point?

68%

34%

34%

Sales

90

100

110

Computing Safety Stock

One Standard Deviation = 68% of the area.

Half of the time sales are below 100; 84% of the time sales are below 110.

95%

47.5%

47.5%

Sales

80

100

120

Computing Safety Stock

Two Standard Deviations = 95% of the area.

Half the time sales are below 100; 97.5% of the time sales are below 120.

Areas under the standard normal probability

distribution between the mean and successive values of z

\\\

\\\\

z0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

0.00.00000.00400.00800.01200.01600.01990.02390.02790.03190.0359

0.10.39800.04380.04780.05170.05570.05960.06360.06750.07140.0753

0.20.07930.08320.08710.09100.09480.09870.10260.10640.11030.1141

0.30.11790.12170.12550.12930.13310.13680.14060.14430.14800.1517

0.40.15540.15910.16280.16640.17000.17360.17720.18080.18440.1879

0.50.19150.19500.19850.20190.20540.20880.21230.21570.21900.2224

0.60.22570.22910.23240.23570.23890.24220.24540.24860.25180.2549

0.70.25800.26120.26420.26730.27040.27340.27640.27940.28230.2852

0.80.28810.29100.29390.29670.29950.30230.30510.30780.31060.3133

0.90.31590.31860.32120.32380.32640.32890.33150.33400.33650.3389

1.00.34130.34380.34610.34850.35080.35310.35540.35770.35990.3621

1.10.36430.36650.36860.37080.37290.37490.37700.37900.38100.3830

1.20.38490.38690.38880.39070.39250.39440.39620.39800.39970.4015

1.30.40320.40490.40660.40820.40990.41150.41310.41470.41620.4177

1.40.41920.42070.42220.42360.42510.42650.42790.42920.43060.4319

1.50.43320.43450.43570.43700.43820.43940.44060.44180.44290.4441

1.60.44520.44630.44740.44840.44950.45050.45150.45250.45350.4545

1.70.45540.45640.45730.45820.45910.45990.46080.46160.46250.4633

1.80.46410.46490.46560.46640.46100.46780.46860.46930.46990.4706

1.90.47130.47190.47260.47320.47380.47440.47500.47560.47610.4767

2.00.47720.47780.47830.47880.47930.47980.48030.48080.48120.4817

2.10.48220.48260.48300.48340.48380.48420.48460.48500.48540.4857

2.20.48610.48640.48680.48710.48750.48780.48810.48840.48870.4890

2.30.48930.48960.48980.59800.49040.49060.49090.49110.49110.4916

2.40.49180.49200.49220.49250.49270.49290.49310.49320.49340.4936

2.50.49380.49400.49410.49430.49450.49460.49480.49490.49510.4952

2.60.49530.49550.49560.49570.49590.49600.49610.49620.49630.4964

2.70.49650.49660.49670.49680.49690.49000.49710.49720.49730.4974

2.80.49740.49750.49760.49770.49770.49780.49790.49790.49800.4981

2.90.49810.49820.49820.49830.49840.49840.49850.49850.49860.4986

3.00.49860.49870.49870.49880.49880.49890.49890.49890.49900.4990

4.00.49997

SDt =

(t)(SD)2 + (D)2 (St)2

Effects 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 time

St = Standard Deviation of delivery time

D =Average Demand

SD =Standard Deviation of Demand

Example

ASSUMPTIONS

Average Sales (D) = 100 units

Standard 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

Example

(Continued)

Combined standard deviations

CARRIER A

SDt = (5) (10)2 + (100)2 (1)2 = 102 units

CARRIER B

SDt = √ (5) (10)2 + (100)2 (2)2 = 201 units

Example

(Continued)

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.

Example

(Continued)

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

Inventory and the Number of Facilities

Basic Trade-off

More facilities = more inventory

More facilities = shorter shipments

Tie in with ABC Analysis

Consolidate slow moving items in central locations

Trade increased transportation for reduced inventory

or vice versa

X

Inventory and the Number of Facilities

$

(Y)

Y= b

Number of Facilities (X)

Joint 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).

Q + I - P

)

(

- Ii + Pi

Qi = Ri

R

Joint Replenishment

Where:

Qi = Order Quantity of the ith item (to be determined)

Ri = Annual Rate of Use of the ith item

Ii = Inventory on hand for the ith item

Pi = Reorder point for the ith item

Q = Total Shipment Size

R = Total of Individual Usage Rate

I = Total of Individual Inventory Items on Hand

P = Total of Individual Reorder Points

Units

Ri Pi Ii

Corn 6,400 163 154

Beans 9,800 218 250

Peas 5,100 150 180

Totals 21,300 531 584

Cases =25 pounds

Ri Pi Ii

Corn160,000 4,075 3,850

Beans245,000 5,450 6,250

Peas127,000 3,750 4,500

Totals 532,50013,27514,600

R P I

Joint Replenishment

Example: Corn, Beans, Peas

Q + I - P

)

(

- Ii + Pi

Qi = Ri

R

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

532,500

Peas:

Qi = 127,500(40,000+14,600-13,275) - 4,500 + 3,750 = 9,145

532,500 40,000

Joint Replenishment

Example: Corn, Beans, Peas