32
PERT 03 Manajemen Persediaan (1) Fungsi Inventory Inventory Management Inventory Model dengan Independent Demand. EOQ Model

PERT 03 Manajemen Persediaan (1) - DINUSdinus.ac.id/repository/docs/ajar/agp-1-03.pdfPERT 03 Manajemen Persediaan (1) Fungsi Inventory InventoryManagement Inventory Modeldengan Independent

Embed Size (px)

Citation preview

PERT 03 Manajemen Persediaan (1)

� Fungsi Inventory

� Inventory Management

� Inventory Model dengan Independent

Demand.

� EOQ Model

What Is Inventory?

� Stock of items kept to meet future demand

� Purpose of inventory management

– how many units to order

– when to order

��1313--22

Types of Inventory

� Raw material

� Work-in-progress

� Maintenance/repair/operating supply

� Finished goods

The Functions of Inventory

� To ”decouple” or separate various parts of the

production process

� To provide a stock of goods that will provide a

“selection” for customers

� To take advantage of quantity discounts

� To hedge against inflation and upward price

changes

The Functions of Inventory (Lanj)

� Bullwhip effect

– demand information is distorted as it moves away

from the end-use customer

– higher safety stock inventories to are stored to

compensate

� Seasonal or cyclical demand

� Inventory provides independence from vendors

� Take advantage of price discounts

� Inventory provides independence between stages

and avoids work stoppages

Two Forms of Demand�� DependentDependent

�� Demand for items used to Demand for items used to produce final products produce final products

� Tires stored at a Goodyear plant are an example of a dependent demand item

�� IndependentIndependent

�� Demand for items used by Demand for items used by external customersexternal customers

� Cars, appliances, computers, and houses are examples of independent demand inventory

Inventory Costs

� Carrying cost

– cost of holding an item in inventory

� Ordering cost

– cost of replenishing inventory

� Shortage cost

– temporary or permanent loss of sales when

demand cannot be met

Holding Costs

� Obsolescence

� Insurance

� Extra staffing

� Interest

� Pilferage

� Damage

� Warehousing

� Etc.

Inventory Holding Costs(Approximate Ranges)

Category

Housing costs

Material handling costs

Labor cost from extra handling

Investment costs

Pilferage, scrap, and obsolescence

Cost as a% of Inventory Value

6%(3 - 10%

3%(1 - 3.5%0

3%(3 - 5%)

11%(6 - 24%)

3%

(2 - 5%)

Ordering Costs

� Supplies

� Forms

� Order processing

� Clerical support

� Etc.

Setup Costs

� Clean-up costs

� Re-tooling costs

� Adjustment costs

� Etc.

The Material Flow Cycle

The Material Flow Cycle (lanj)

� Run time: Job is at machine and being worked on

� Setup time: Job is at the work station, and the work station

is being "setup."

� Queue time: Job is where it should be, but is not being

processed because other work precedes it.

� Move time: The time a job spends in transit

� Wait time: When one process is finished, but the job is

waiting to be moved to the next work area.

� Other: "Just-in-case" inventory.

�Higher costs– Item cost (if purchased)

– Ordering (or setup) cost• Costs of forms, clerks’ wages etc.

– Holding (or carrying) cost• Building lease, insurance, taxes etc.

�Difficult to control

�Hides production problems

Disadvantages of Inventory

� Divides on-hand inventory into 3 classes

– A class, B class, C class

� Basis is usually annual $ volume

– $ volume = Annual demand x Unit cost

� Policies based on ABC analysis

– Develop class A suppliers more

– Give tighter physical control of A items

– Forecast A items more carefully

ABC Analysis

0

20

40

60

80

100

0 50 100

% of Inventory Items% of Inventory Items

% Annual $ Usage% Annual $ Usage

AA

BBCC

Class % $ Vol % ItemsA 80 15B 15 30C 5 55

Classifying Items as ABC

ABC Classification: Example

��1313--1717

11 $ 60$ 60 9090

22 350350 4040

33 3030 130130

44 8080 6060

55 3030 100100

66 2020 180180

77 1010 170170

88 320320 5050

99 510510 6060

1010 2020 120120

PARTPART UNIT COSTUNIT COST ANNUAL USAGEANNUAL USAGE

ABC Classification: Example (cont.)

��1313--1818

Example 10.1Example 10.1

11 $ 60$ 60 9090

22 350350 4040

33 3030 130130

44 8080 6060

55 3030 100100

66 2020 180180

77 1010 170170

88 320320 5050

99 510510 6060

1010 2020 120120

PARTPART UNIT COSTUNIT COST ANNUAL USAGEANNUAL USAGETOTAL % OF TOTAL % OF TOTALPART VALUE VALUE QUANTITY % CUMMULATIVE

9 $30,600 35.9 6.0 6.08 16,000 18.7 5.0 11.02 14,000 16.4 4.0 15.01 5,400 6.3 9.0 24.04 4,800 5.6 6.0 30.03 3,900 4.6 10.0 40.06 3,600 4.2 18.0 58.05 3,000 3.5 13.0 71.0

10 2,400 2.8 12.0 83.07 1,700 2.0 17.0 100.0

$85,400

AA

BB

CC

% OF TOTAL % OF TOTALCLASS ITEMS VALUE QUANTITY

A 9, 8, 2 71.0 15.0B 1, 4, 3 16.5 25.0C 6, 5, 10, 7 12.5 60.0

Techniques for Controlling Service Inventory Include:

�Good personnel selection, training, and

discipline

�Tight control of incoming shipments

�Effective control of all goods leaving the

facility

� Fixed order-quantity

models

– Economic order quantity

– Production order quantity

– Quantity discount

� Probabilistic models

� Fixed order-period models

Help answer the

inventory planning

questions!

Help answer the

inventory planning

questions!

© 1984-1994 T/Maker Co.

Inventory Models

� Known and constant demand

� Known and constant lead time

� Instantaneous receipt of material

� No quantity discounts

� Only order (setup) cost and holding cost

� No stockouts

EOQ Assumptions

Order QuantityOrder Quantity

Annual CostAnnual Cost

Holding

Cost C

urve

Holding

Cost C

urve

Total Co

st Curve

Total Co

st Curve

Order (Setup) Cost CurveOrder (Setup) Cost Curve

Optimal Optimal

Order Quantity (Q*)Order Quantity (Q*)

EOQ Model, How Much to Order?

�More units must be stored if more are ordered

Purchase Order

Description Qty.

Microwave 1

Order quantityOrder quantity

Purchase Order

Description Qty.

Microwave 1000

Order quantityOrder quantity

Why Holding Costs Increase

� Cost is spread over more units

Example: You need 1000 microwave ovens

Purchase Order

Description Qty.

Microwave 1

Purchase Order

Description Qty.

Microwave 1

Purchase Order

Description Qty.

Microwave 1

Purchase OrderDescription Qty.Microwave 1

1 Order (Postage $ 0.33)1 Order (Postage $ 0.33) 1000 Orders (Postage $330)1000 Orders (Postage $330)

Order quantityOrder quantity

Purchase Order

Description Qty.Microwave 1000

Why Order Costs Decrease

Deriving an EOQ

� Develop an expression for setup or

ordering costs

� Develop an expression for holding cost

� Set setup cost equal to holding cost

� Solve the resulting equation for the best

order quantity

Inventory Order Cycle

��1313--2626

Demand Demand raterate

TimeTimeLead Lead timetime

Lead Lead timetime

Order Order placedplaced

Order Order placedplaced

Order Order receiptreceipt

Order Order receiptreceipt

Inven

tory

Le

vel

Inven

tory

Le

vel

Reorder point, Reorder point, RR

Order quantity, Order quantity, QQ

00

Average Average

inventoryinventory

QQ

22

EOQ Cost Model

��1313--2727

CCoo -- cost of placing ordercost of placing order DD -- annual demandannual demand

CCcc -- annual perannual per--unit carrying costunit carrying cost QQ -- order quantityorder quantity

Annual ordering cost =Annual ordering cost =CCooDD

QQ

Annual carrying cost =Annual carrying cost =CCccQQ

22

Total cost = +Total cost = +CCooDD

QQ

CCccQQ

22

EOQ Cost Model

��1313--2828

TC = +CoD

Q

CcQ

2

= – +CoD

Q2

Cc

2

∂∂∂∂TC

∂∂∂∂Q

0 = – +C0D

Q2

Cc

2

Qopt =2C

oD

Cc

Deriving Qopt Proving equality of costs at optimal point

=CoD

Q

CcQ

2

Q2 =2C

oD

Cc

Qopt =2C

oD

Cc

EOQ Cost Model (cont.)

��1313--2929

Order Quantity, Order Quantity, QQ

Annual Annual cost ($)cost ($) Total CostTotal Cost

Carrying Cost =Carrying Cost =CCccQQ

22

Slope = 0Slope = 0

Minimum Minimum total costtotal cost

Optimal orderOptimal orderQQoptopt

Ordering Cost =Ordering Cost =CCooDD

QQ

EOQ Example

��1313--3030

CCcc = $0.75 per gallon= $0.75 per gallon CCoo = $150= $150 DD = 10,000 gallons= 10,000 gallons

QQoptopt ==22CCooDD

CCcc

QQoptopt ==2(150)(10,000)2(150)(10,000)

(0.75)(0.75)

QQoptopt = 2,000 gallons= 2,000 gallons

TCTCminmin = += +CCooDD

QQ

CCccQQ

22

TCTCminmin = += +(150)(10,000)(150)(10,000)

2,0002,000

(0.75)(2,000)(0.75)(2,000)

22

TCTCminmin = $750 + $750 = $1,500= $750 + $750 = $1,500

Orders per year =Orders per year = DD//QQoptopt

== 10,000/2,00010,000/2,000

== 5 orders/year5 orders/year

Order cycle time =Order cycle time = 311 days/(311 days/(DD//QQoptopt))

== 311/5311/5

== 62.2 store days62.2 store days

EOQ Model, When To Order

Reorder Reorder

Point (ROP)Point (ROP)

TimeTime

Inventory LevelInventory Level

AverageAverage

Inventory Inventory

(Q*/2)(Q*/2)

Lead TimeLead Time

Optimal Optimal

Order Order

QuantityQuantity

(Q*)(Q*)

Optimal Order Quantity

Expected Number of Orders

Expected Time Between Orders Working Days / Year

Working Days / Year

= =× ×

= =

= =

=

= ×

Q*D S

H

ND

Q*

TN

dD

ROP d L

2

DD = Demand per year= Demand per year

SS = Setup (order) cost per order= Setup (order) cost per order

HH = Holding (carrying) cost = Holding (carrying) cost

dd = Demand per day= Demand per day

LL = Lead time in days= Lead time in days

EOQ Model Equations