存貨管理. (inventory management) inventories are materials and supplies that a business or...
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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
存貨管理(Inventory Management)
Production Planning
Master Planning
Material RequirementPlanning
Overall Inventory
End Items Inventory
Component Parts andRaw Material
存貨管理(Inventory Management)
•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)
存貨管理(Inventory Management)
•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.
存貨管理(Inventory Management)
•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
存貨管理(Inventory Management)
•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
存貨管理(Inventory Management)
•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?
存貨管理(Inventory Management)
•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
存貨管理(Inventory Management)
•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
存貨管理(Inventory Management)
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
存貨管理(Inventory Management)
•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.
存貨管理(Inventory Management)
•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.
存貨管理(Inventory Management)
•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
存貨管理(Inventory Management)
•Development of the EOQ Formula
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
存貨管理(Inventory Management)
•Development of the EOQ Formula
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
存貨管理(Inventory Management)
•Development of the EOQ Formula
Total Cost
Ordering Cost
Carrying Cost
Cos
t in
Dol
lars
Lot Size
存貨管理(Inventory Management)
•Development of the EOQ Formula
ci
SAQ
Q
SAciQ
2
2
Cost OrderingCost Carrying
存貨管理(Inventory Management)
•How to Reduce Lot Size
ci
SAQ
2
Unit Cost Increase
Order Cost decrease
存貨管理(Inventory Management)
•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?
存貨管理(Inventory Management)
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.
存貨管理(Inventory Management)
•Order Point System
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 )
存貨管理(Inventory Management)
•Order Point System
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
存貨管理(Inventory Management)
•Order Point System
Qua
ntit
y
TimeLead Time
Safety Stock
Order Point
Expected demand during lead time
Maximum probabledemand during lead time
存貨管理(Inventory Management)
•Order Point System
Expected Demand
OrderPoint
Safety Stock
Service Level(Probability ofno stock out)
Risk of a Stock-out
0 z Z-scale
存貨管理(Inventory Management)
•Order Point System
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.
存貨管理(Inventory Management)
•Order Point System
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
存貨管理(Inventory Management)
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
存貨管理(Inventory Management)
•Order Point System
•Order Point System
If both demand and lead time are variable, then
222LTddLT dLT
, and the reorder point is
222LTd dLTzLTdOP
存貨管理(Inventory Management)
•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
存貨管理(Inventory Management)
The actual demand will be within 1 MAD of the forecast averageabout 60% of the time.
The actual demand will be within 2 MAD of the forecast averageabout 90% of the time.
The actual demand will be within 3 MAD of the forecast averageabout 98% of the time.
•Order Point System - An Alternative Way
存貨管理(Inventory Management)
•Order Point System - An Alternative Way
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
存貨管理(Inventory Management)
•Order Point System - Determining When the Order Point is Reached
Two-Bin System
Perpetual Inventory Record System
存貨管理(Inventory Management)
•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
存貨管理(Inventory Management)
•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
存貨管理(Inventory Management)
存貨管理(Inventory Management)
•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 .
存貨管理(Inventory Management)
•Noninstantaneous Receipt
存貨管理(Inventory Management)
•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?
存貨管理(Inventory Management)
•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
存貨管理(Inventory Management)
•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
存貨管理(Inventory Management)
•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
存貨管理(Inventory Management)
•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
存貨管理(Inventory Management)
•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.
存貨管理(Inventory Management)
•Periodic Review System
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
•Periodic Review System
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.
存貨管理(Inventory Management)
存貨管理(Inventory Management)
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