CHAPTER 9
Inventory Management
Five Learning Objectives
 To determine (計算)the costs of holding inventory
 To identify the costs associated with a stockout
 To understand the EOQ concept
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 To differentiate the various inventory flow patterns
 To appreciate the role of scanners in inventory
control
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17 Key Terms
 ABC analysis
 Economic order
quantity (EOQ)
 Fixed order interval
system定期訂購系統
 Fixed order quantity
system定量訂購系統
 Handling costs
 Insurance costs
 Inventory carrying
(holding) costs存貨
(持有)成本
 Inventory shrinkage
存貨損耗
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 Marginal analysis邊際
效益分析
 Obsolescence
 Opportunity cost
 Reorder point (ROP)
 Safety stocks
 Stockouts
 Storage costs儲存成本
 Taxes
 Vendor-managed
inventory (VMI)供應商
庫存管理
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content
 9.1 Inventory Classifications
 9.2 Inventory-Related Costs
 9.3 When to Order
 9.4 How Much to Reorder
 9.5 Inventory Flows
 9.6 Contemporary (當代)Approaches to Managing
Inventory
 9.7 Inventory Management: Special Concerns
9-6
Inventory Management
 Inventories are stocks of goods and materials that
are maintained to satisfy normal demand patterns
 Inventory management
 Decisions drive other logistics activities
 Different functional areas have different inventory
objectives
Ex: finance vs. marketing dept.
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 Inventory costs are important to consider
 Inventory turnover: cost of goods sold divided by average
inventory at cost
cost of goods sold = inventory turnover
average inventory
Ex:
$200,000 = inventory is sold 4 times per year…愈大愈好
$ 50,000
 Compare with competitors or benchmarked companies
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 Low inventory turnover = high inventory carrying
costs, little (or no) stockout costs
 High inventory turnover = low inventory carrying
costs, high stockout costs
 Managing the tradeoff is important to maintain
service levels
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9.1 Inventory Classifications
 Psychic stock 精神庫存(stimulates demand!):如零
售店架上存貨、看到會想買！
 Cycle or base stock週期/基礎庫存：為一般需求之
採買
 Safety or buffer stock安全/緩衝庫存：週期存貨外、
應付不確定需求之存貨
 Pipeline or in-transit stock管線/過渡庫存：供應鏈
上不同倉庫間之存貨
 Speculative stock推測庫存：應付季節性需求、漲
價預期等之存貨
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9.2 Inventory-Related Costs
 Inventory carrying (holding) costs：包含下列因素








Obsolescence過時
Inventory shrinkage：入庫數大於出庫數
Storage costs：如空間成本
Handling costs
Insurance costs
Taxes………通常當天的存貨要壓低
Interest charges利息成本
Opportunity cost＝備料錯誤之成本
 Stockouts (costs)…as shown in table 9-1
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Table 9-1: Determination of the
Average Cost of a Stockout
Alternative(種類)
1. Brand-loyal customer
2. Switches(向別家買)
and comes
back
3. Lost customer(損失顧客)
Average cost of a
stockout
Loss
Probability
$00.00
.10
$37.00
.65
$1,200
Average Cost
$00.00
$24.05
.25
$ 300.00
1.00
$324.05
These are hypothetical figures (假設數字)for illustration.
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 由前表可知，Stockouts (costs)愈高之品、需
要準備愈多之safety stock
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 Trade-offs exist between carrying and
stockout costs
 Marginal analysis：找出讓存貨持有成本與缺貨成
本相同之(安全)存貨水準!
 As shown in table 9-2
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Table 9-2: Determination of
Safety Stock Level
Number of Units
of Safety Stock
Total Value of
Safety Stock
($480 per Unit)
25%
Annual
Carrying
Cost
Carrying
Cost of
Incremental
Safety Stock
Number of
Additional
Orders
(Filled)
Additional
Stockout
Costs
Avoided(節
省)
10
$4,800
$1,200
$1,200
20
$6,481.00
20
9,600
2,400
1,200
16
5,184.80
30
14,400
3,600
1,200
12
3,888.60
40
19,200
4,800
1,200
8
2,592.40
50
24,000
6,000
1,200
6
1,944.30
60
28,800
7,200
1,200
4
1,296.20
70
33,600
8,400
1,200
3
972.15
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9.3 When to Order
 Fixed order quantity system
 Fixed order interval system
 Reorder point (ROP)
ROP = DD x RC
under certainty
ROP = (DD x RC) + SS under uncertainty
Where DD = daily demand
RC = length of replenishment cycle
SS = safety stock
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9.4 How Much to Reorder
 Economic order quantity (EOQ) in dollars
EOQ = √(2AB/C)
Where
EOQ = the most economic order size, in
dollars
A = annual usage(用量), in dollars
B = administrative costs per order of placing
the order
C = carrying costs of the inventory (%：每年存
貨成本之百分比)
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 Economic order quantity (EOQ) in units
EOQ = √(2DB/IC)
Where
EOQ = the most economic order size, in units
A = annual demand, in units
B = administrative costs per order of placing the
order
C = carrying costs of the inventory (%)
I = dollar value of the inventory, per unit
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Figure 9-2: Determining EOQ
by Use of a Graph
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Table 9-3: EOQ Cost
Calculations
Number of
orders (訂
購)per year
Order size
($)
1
1,000
2
Ordering cost
($)
Carrying cost
($)
Total cost (sum of ordering
and carrying cost) ($)
25
100
125
500
50
50
100
3
333
75
33
108
4
250
100
25
125
5
200
125
20
145
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 簡單的EOQ公式無考慮購量大之下的折扣因素
 EOQ中假設需求是連續、不受時間影響的（確定的
需求模式！）
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9.5 Inventory Flows
 Safety stock can prevent against two problem
areas
 Increased rate of demand
 Longer-than-normal replenishment(補貨延誤)
 如後圖
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Figure 9-3: Inventory Flow Diagram
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 由前圖可知，若原本在第十四天會到的貨延至
第十五天才到，因此第十四天的30單位貨由安
全存貨支應（M點），隨後EOQ到貨
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 When fixed order quantity system like EOQ is used,
time between orders may vary
 When reorder point is reached, fixed order quantity
is ordered……電腦化設定reorder point !
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9.6 Contemporary (當代)Approaches
to Managing Inventory
 ABC Analysis
有些企業會加入D等級貨、像是dogs（看門狗）或
dead inventory（呆貨）之意味：表已無需求之存貨、
只增加存貨持有成本！
使用ABC Analysis需謹記： A等級之品不可過多！否
則分類管理之技巧便無意義
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 Just-in Time (JIT) Approach：將存貨視為浪費(注重
原物料流動！), ex: supplier park
Just-in-case (JIC)：將存貨視為應變突發需求之保障
Lean inventory（臨界存貨、精敏存貨）：著重製造
商與零售商間產品、存貨流動。如ECR（用於飲料
業、零售業）與QR（用於成衣業）
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 Vendor-Managed Inventory (VMI):供應商決定
下單的時間。如螺絲業、消費品業
需避免共享資料不正確之問題…..
 Inventory Tracking:如利用無線射頻辨識
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9.7 Inventory Management:
Special Concerns
 Defining stock-keeping units (SKUs)
材料需以SKU或LINE ITEMS（線上貨）做分類！
不同SKU:代表種類不同或記錄方式不同之貨
 Dead inventory……失敗的新產品？
 Deals(交易)：指將暢銷品與滯銷品組合出售
 Substitute items：企業需注意某些替代關係是單向的。如小螺
栓可能取代大螺栓、反之卻不然
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 Complementary items(搭配品)：指需一起使用、一
起銷售之貨
 Informal arrangements outside the distribution
channel:如連鎖店間調貨
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 Repair/replacement parts：如有公司提供
parts bank（零件銀行）服務！
特例：零件製造商給採購量最多之伙伴免費度假
旅遊。然下游伙伴為取悅其妻、增加不必要採
購，造成供應鏈中該零件庫存大量增加
 Reverse logistics……回退品與回收品（可再
利用者）
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981小考二題庫
 何謂ABC存貨分析?
 存貨持有成本包含哪8個主要因子？
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