存貨管理. (inventory management) inventories are materials and supplies that a business or...

存貨管理

存貨管理(Inventory Management)

Inventories are materials and supplies that a business orinstitution carries either for sale or to provide inputs orsupplies to the production process.

原物料 (raw material) ，物料 (supplies) ，半成品 (WIP) ，完成品 (finished goods)

Inventories

•Introduction


Production Planning

Master Planning

Material RequirementPlanning

Overall Inventory

End Items Inventory

Component Parts andRaw Material


•Inventory and the Flow of Material

Warehouse

Customer Demand Customer Demand Customer Demand

Warehouse Warehouse

Finished Goods

Work In Process

Raw M aterialsPurchased Parts and M aterials

Suppliers

Distribution Inventory

Maintenance, Repair, andOperational Supplies(MROs)


•Function of Inventories

Anticipation Inventory: These inventories are built up in anticipation of future demand

Fluctuation Inventory: Inventory is held to cover random unpredictable fluctuation in supply and demand or lead time

Lot-Size Inventory: Items purchased or manufactured in quantities greater than needed immediately create lot-size inventories

Transportation Inventory: These inventories exist because of the time needed to move goods from one location to another.


•Objectives of Inventory Management

減少存貨投資

資金靈活運用

開工水準安定化

減少存貨成本

防止生產遲延及缺料


•Inventory Costs

Item Cost: direct material, direct labor, factory overhead transportation, insurance

Carrying Cost: capital cost, storage cost, risk cost

Ordering Cost: 請購，審核，郵寄，跟催，比價

Stockout Cost: back-order, lost sales, lost customers


•ABC Inventory Control

Control of inventory is exercised by controlling individual items calledstock-keeping units (SKUs). In controlling inventory, four questionsmust be answered:

•What is the importance of the inventory item?•How are they to be controlled?•How much should be ordered at one time?•When should an order be placed?


•ABC Inventory Control

The ABC principle is (Pareto’s Law) based on the observation that a small number of items often dominate the results achieved in any situation. As applied to inventories, it is usually found that the relationshipbetween the percentage of items and the percentage of annualdollar usage follows a pattern in which

A About 20% of the items account for about 80% of the dollar usageB About 30% of the items account for about 15% of the dollar usageC About 50% of the items account for about 5% of the dollar usage


•Step in Making an ABC Analysis

Step 1: Establish the item characteristics that influence the results of inventory management. This is usually annual dollar usage but may be other criteria, such as scarcity of material.

Step2 : Classify items into groups based on the established criteria.

Step3 : Apply a degree of control in proportion to the importance of the group.

•Have plenty of low-value items

•Use the money and control effort saved to reduce the inventory of high-value items


A 料件

1. 經常對需求預測與預測方法作評估2. 經常週期性進行嚴格的盤點3. 每日更新記錄4. 經常對需求之需要、訂購量與安全存量進行檢視5. 嚴密跟催以縮短前置時間

B 料件與 A 料件相同，但管制頻率較低

C 料件1. 基本原則在保有此種存貨2. 非經常性地進行較不嚴格之盤點


•Lot Size Decision Rules

Lot for Lot - To order exactly what is needed (no more no less). The order quantity changes whenever requirements change.

Fixed-order quantity - The number of units to be ordered each time is fixed. The quantity is usually arbitrary.

Min-Max System - An order is placed when the quantity available falls below the order point.

Period-Order quantity - Order enough to satisfy future demand for a given period of time.

Costs- Minimize the sum of ordering cost and carrying cost.


•Economic-Order Quantity (EOQ)

Assumptions

1. Demand is relatively constant and is known.

2. The item is produced or purchased in lots or batches and not continuously.

3. Order preparation costs, inventory-carrying costs, and lead times are constant and known.

4. Replacement occurs all at once.


•Development of the EOQ Formula

Units In Stock

Time

Q=Lot Size

•Average inventory = Order Quantity/2•Number of order per year = annual demand/order quantity



Relevant costs

•Annual cost of placing orders•Annual cost of carrying inventory

Let A = annual usage in units S = ordering cost in dollars per order i = annual carrying cost rate as a decimal of a percentage c = unit cost in dollars Q = order quantity in units



costs carrying annual costs ordering annual cost annual total2

ratecost carryingcostunit inventory average

year onefor

unit one carrying ofcost inventory average cost carrying annual

orderper cost orders ofnumber cost ordering annual

icQ

SQ

A



Total Cost

Ordering Cost

Carrying Cost

Cos

t in

Dol

lars

Lot Size



ci

SAQ

Q

SAciQ

2

2

Cost OrderingCost Carrying


•How to Reduce Lot Size

ci

SAQ

2

Unit Cost Increase

Order Cost decrease


•Monetary Unit Lot Size ( 貨幣單位批量 ) - EOQ in Dollar

AD = annual usage in dollars

S = ordering costs in dollars

i = carrying cost rate as a decimal of a percent

i

SAc

ci

SAcEOQEOQ D

22

'

Order Point System

Periodic Review System

Independent Demand

When to place a replacement order?


Material Requirement Planning

Dependent Demand

•Order Point System

Units in Stock

Q

Lead Time

Safety Stock

Order Point

Note: Q is usually precalculated and based on economic-order quantity.



OP=DDLT+SS

Where OP=Order point DDLT=demand during the lead time SS=safety stock

Average inventory = order quantity / 2 + safety stock

Determine the order point depends on the demand duringthe lead time and the safety stock

When variability is present in demand or lead time, the possibilitythat actual demand will exceed expected demand is created. Consequently, it becomes necessary to carry additional inventory, called Safety Stock ( 安全庫存 )



The amount of safety stock that is appropriate for a given situation dependson the following factors

1. The average demand rate and average lead time2. Demand and lead time variability3. The desired service level

Service level = 100 percent - Stock-out risk

Order cycle service level can be defined as the probability that demandwill not exceed supply during lead time. Hence, a service level of 95%implies a probability of 95% that demand will not exceed supply duringlead time



Qua

ntit

y

TimeLead Time

Safety Stock

Order Point

Expected demand during lead time

Maximum probabledemand during lead time



Expected Demand

OrderPoint

Safety Stock

Service Level(Probability ofno stock out)

Risk of a Stock-out

0 z Z-scale



dLTσz timelead during demand expectedOP

Where z = number of standard deviation

dLT = The standard deviation of lead time demand

The models generally assume that any variability in demand rate or lead time can be adequately described by a normal distribution.



When data on lead time demand are not readily available, the previousformula cannot be used.

If only demand is variable, then ddLT LT , and the reorder

point is

dLTzLTdOP

where

or weekday per demand ofdeviation Standard

demandor weekly daily Average

d

d


If only lead time is variable, then LTdLT d , and the reorder

point is

where

or weeks daysin timelead ofdeviation Standard

or weeks daysin timelead Average

LTσ

LT

LTdzLTdOP




If both demand and lead time are variable, then

222LTddLT dLT

, and the reorder point is

222LTd dLTzLTdOP


•Order Point System - An Alternative Way

nsobservatio ofnumber

deviations absolute of sumMAD

30%30%

15% 15%

4%4%

1%1%

0 +1 +2 +3-1-2-3

MAD


The actual demand will be within 1 MAD of the forecast averageabout 60% of the time.






Safety Stock = MAD safety factor

Service Level Safety factor50 0.0075 0.8480 1.0585 1.3090 1.6094 1.9595 2.0696 2.1997 2.3598 2.5699 2.91

99.5 3.2099.99 5.00


•Order Point System - Determining When the Order Point is Reached

Two-Bin System

Perpetual Inventory Record System


•Order Point System - Perpetual Inventory Record System

Permanent Information : Part Number Storage Location Order Point Order Quantity Lead Time Safety Stock Suppliers

Variable Information : Quantities ordered: dates, order number, and quantity Quantities received: dates, order number, and quantity Quantities issued Balance on hand Allocated Available balance


•Order Point System - Perpetual Inventory Record System

Ex:

42654 Screw Order Quantity500

Order Point100

Date Ordered Received Issue On Hand Allocated Available01 0 0 0 500 0 50002 0 0 0 500 400 10003 500 0 0 500 0 10004 0 0 400 100 0 10005 0 500 0 600 0 600



•Noninstantaneous Receipt

Q=lot sizeQm

Tp

Td

)1( ;

)(

)(

p

dQQ

T

Qd

T

Qp

T

TTQQ

TT

Q

T

Q

mdp

d

pdm

pd

m

d

)1(

2

pd

ci

SAEOQ

Where A, S, c, and i have the same meanings as before and d = usage rate p = production rate .


•Noninstantaneous Receipt


•Quantity Discount

Example: An item has an annual demand of 25000 units, a unit cost of $10, an order preparation cost of $10, and a carrying cost of 20%. It is ordered on the basis of an EOQ, but the supplier has offered a discount of 2% on orders of $10000. Shouldthe offer be accepted?


•Use of EOQ When Costs are Not Known

Example: Suppose there were a family of items for which the decision rule was to order each item four times a year. Since the cost of ordering and the cost of carrying inventory are not known, ordering four times a year is not based on an EOQ. Can we come up with a better decision rule even if the EOQ cannot be calculated?

Item Annual Usage Orders per Year

ABC

10,000 400 144

444


•Use of EOQ When Costs are Not Known

Assume S and i are the same for all items, the ratio of 2S i must be the same forall items in the family.

N

AkTherefore

N

AAkQThen

i

SkLet

i

SAQ

D

DD

D

2

2


•Period Order Quantity (POQ)

The economic-order quantity attempts to minimize the total cost of ordering andcarrying inventory and is based on the assumption that demand is uniform. Oftendemand is not uniform, particularly in material requirement planning, and usingthe EOQ does not produce a minimum cost.The period-order quantity lot-size rule is based on the same theory as the economic-order quantity. It uses the EOQ formula to calculate an economic timebetween orders.

usage weekly average

EOQPOQ


•Period Order Quantity (POQ)

Example: Given the following MRP record and an EOQ of 250 units, calculate the planned order receipts using the economic-order quantity. Next, calculate the period-order quantities and the planned order receipts. In both cases, calculate the ending inventory and the total inventory carried over the ten weeks.

Week 1 2 3 4 5 6 7 8 9 10 TotalNet

Requirements100 50 150 75 200 55 80 150 30 890

Planned OrderReceipt


•Periodic Review System

The period review system is useful for the following:

Where there are many small issues from inventory, and postingtransactions to inventory records are very expensive.

Where ordering cost are small. This occurs when many different items are ordered from one source.

Where many items are ordered together to make up a productionrun or fill a truckload.



Using the periodic review system, the quantity on hand of a particularitem is determined at specified, fixed time intervals, and an order isplaced.

R1 R2 R3 R4

t1 t2 t3

Q1Q2 Q3 The review interval (t1,t2, and t3)

are equal and that Q1, Q2, and Q3are not necessary the same.

Review period


T = target (maximum) inventory levelD = demand per unit of timeL = lead time durationR = review period durationSS = safety stock

SSLRDT )(

Note: The quantity equal to the demand during the lead time plus the demand during the review period plus safety stock is called the target level or maximum-level inventory.



Periodic Review System

Order amount = T – On-hand Inventory – Earlier orders not yet received + Back order

物料主檔

倉儲作業系統

庫存紀錄庫存數字之記錄方式，因管理需求之不同可以區分

成：存貨數量之記錄。存貨價值之記錄。

Note: 存貨價值 = 存貨數量 x 單位成本

倉儲作業系統

庫存記錄的主要內容庫存數量 : 庫存良品數 .可用庫存量 : 庫存數量扣除待出貨數量 , 即為可出貨之數量 .待出貨量 : 已經申請 ( 通知 ) 出貨 , 但是尚未出貨之數量 .訂單餘量 : 接到客戶之訂單而尚未交貨之數量 .請購數量 : 已經提出請購單 , 但尚未正式下採購單之數量 .待產數量 : 已經安排工令單資料 , 但尚未完成生產並繳庫的數量 .已分配量 : 已經安排將要領出銷貨 , 或將被領出生產的數量 .在製數量 : 製造現場已經領出 , 但尚未完工繳回之廠內在製品數 .外包數量 : 外包廠商已經領出 , 但尚未完工繳回之委外加工品數 .已購未入 : 已經下採購單 , 尚未驗收入庫的數量 .

庫存紀錄的變更

事件異動對象庫存欄位變化下訂單成品訂單餘量增加

MRP展開成品、半成品原物料

待產數量增加請購數量增加已分配量增加

轉採購單原物料請購數量減少已購未入增加

採購驗收原物料庫存數量增加已購未入減少可用庫存量增加

發料半成品原物料

庫存數量減少已分配量減少可用庫存量減少在製或外包數量增加

生產完成入庫成品、半成品可用庫存量增加庫存數量增加待產數量減少在製或外包數量減少

申請出貨成品待出貨量增加可用庫存量減少

出貨成品庫存數量減少訂單餘量減少待出貨量減少

倉儲作業系統

呆廢料其他管理事項自行再加工。調撥使用。拼修重組。拆零利用。出售或交換。銷毀。

倉儲作業系統

報廢作業與庫存紀錄庫存物料報廢

庫存數量減少可用庫存量減少報廢數量增加

在製品報廢在製數量減少報廢數量增加已分配量增加

Note: 報廢數量指的是在報廢倉庫理的物料

存貨管理. (inventory management) inventories are materials and supplies that a business or...

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