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Konsep Dasar Persediaan
Outline:1. What is Inventory?2. Key Factors in Inventory Management
Decision3. Financial Inventory Performances4. ABC Analysis.
Kuliah ke-6: Rabu, 22 Okt 2008
I. What is Inventory (Persediaan)?
• Inventories:
“…. are materials and supplies that a business carries either for sale or to provide inputs or supplies to the production process.”
• Inventory management:
“….is responsible for planning and controlling inventory from the raw material stage to the customer.”
• Since inventory either results from production or supports it, it cannot be managed separately, and therefore, must be coordinated.
(1). Waktu operasi (run time): Pekerjaan sedang dalam pengerjaan mesin. (2). Waktu persiapan (setup time): Pekerjaan pada stasiun kerja, dan stasiun kerja sedang
“dipersiapkan.“(3). Waktu antri (queue time): Pekerjaan antri sebelum diproses, menunggu pekerjaan yang
mendahuluinya.(4). Waktu pemindahan (move time): Waktu suatu pekerjaan berada dalam transit.(5). Waktu menunggu (wait time): Waktu pekerjaan menunggu untuk dipindahkan ke areal
kerja berikutnya.(6). Lain-lain (other): "Just-in-case" inventory.
– Siapkan jumlah pemasok kelas A yang lebih banyak– Lakukan kontrol fisik lebih ketat untuk item A – Lebih hati-hati meramalkan item A.
Kinerja: Siklus Aliran Material
other (6) waittime (5)
movetime (4)
queuetime (3)
setuptime (2)
runtime (1)Input
Cycle Time
Output
Contoh Inventory
• Stok material• Cadangan kapasitasContoh:
II. Key Factors in Inventory Management Decision
Key Factors in Inventory Management Decision:1. Inventory management objectives2. Flow of material3. Pola supply dan demand4. Inventory functions5. Inventory costs.
2.1. Tujuan Inventory Management
1) Maximum customer service:• Percentage of orders shipped on schedule• Percentage of line items shipped on schedule• Order-days out of stock• etc.
2) Minimum inventory investments
3) Low-cost plant operation:• Memungkinkan operasi berurutan dengan
production rate yang berbeda• Memungkinkan leveling production• Memungkinkan pembelian partai besar dengan order
cost rendah dan quantity discounts• etc.
2.2. Flow of material
Jenis inventory berdasarkan flow of materialterdiri dari:1. Raw materials2. Work-in-process (WIP)3. Finished goods4. Distribution inventories5. Maintenance, repair, operational
supplies (MRO’s)
Contoh: Berdasarkan aliran material
Persediaan(Inventory)
AliranMaterial
Raw Mat'lWIPFin. Goods
.
Industri Komputer
Contoh: Berdasarkan Kuantitas & Nilai
Persediaan(Inventory)
AliranMaterial
Kuantitas& Nilai
Raw Mat'lWIP
Fin. Goods
A ItemsB ItemsC Items
AA
BBCC
Industri Otomotif
Contoh: Berdasarkan Jenis Permintaan
Persediaan(Inventory)
AliranMaterial
JenisPermintaan
Kuantitas& Nilai
Raw Mat'lWIP
Fin. Goods
IndependentDependent
Item AItem BItem C
Independent
Dependent
Contoh: MROs
Persediaan
AliranMaterial
JenisPermintaan
Kuantitas& Nilai Lain-lain
Raw Mat'lWIP
Fin. Goods
IndependentDependent
Item AItem BItem C
Mainten.Repair
Operating
(Inventory)
2.3. Pola Supply & Demand
1. Inventory tidak diperlukan, bila supply bisa memenuhidemand. Kondisi yang diperlukan:• Demand yang stabil, predictable, dan konstan untuk
jangka panjang.• Production rate dibuat sama dengan demand rate
2. Kondisi keseimbangan antara supply dan demand dapatdidekati, bila produksi dapat dilakukan dengan line-flow system.
3. Kondisi keseimbangan antara produksi dengan demand sulitdicapai, karena:• Biasanya produksi dalam lot atau batch• Ada pemisahan antara areal mesin, perakitan dan welding• Pekerjaan bergerak dari satu workstation ke workstation
lainnya.
2.4. Inventory Functions
1. Anticipatipation inventory2. Fluctuation inventory (safety stock)3. Lot size inventory4. Transportation inventory5. Hedge inventory6. MROs inventory
2.5. Total Cost of Inventory
Inventory cost components
1. Item cost: price of item, incl. transportation cost, custom and duties, and insurance (landed costs)
2. Carrying cost: incl. capital costs, storage costs, risk costs (obsolescence, damage, pilferage, deterioration)
3. Ordering cost: incl. production control costs, setup and teardown costs, lost capacity cost, purchase order cost.
4. Stockout cost: could be backorder costs, lostssales, and possibly lost customers.
5. Capacity-associated costs: for changing output levels, may be needed costs for overtime, hiring, training, extra shifts, and layoffs.
Terminologi Biaya Persediaan
A = Pemakaian per tahun (unit)S = Biaya pemesanan ($ per pemesanan)i = rate biaya penyimpanan per tahun
(% atau desimal)c = Biaya per unit ($)Q = Kuantitas pemesanan (unit)
Penghitungan Biaya Persediaan
1. Biaya pemesanan per tahun (annual ordering cost) =Frekuensi pemesanan x Biaya per pemesanan= A/Q x S
2. Biaya penyimpanan per tahun (annual carrying cost)= Rata-rata persediaan x biaya penyimpanan
satu unit per tahun= Rata-rata persediaan x biaya per unit x
biaya penyimpanan= Q/2 x c x i
3. Biaya total per tahun = Biaya pemesanan per tahun + Biaya penyimpanan per tahun
= (A/Q x S) + (Q/2 x c x i)
Contoh penghitungan inventory costs
Annual demand 10,000 unit, ordering cost $30 per order, the carrying cost 20%, and the unit cost $15. Order quantity 600 unit.Maka:A = 10,000 unitS = $30i = 0.20c = $15Q = 600 unit.Jadi:1. Annual ordering cost = A/Q x S = 10000/600 x $30= $5002. Annual carrying cost = Q/2 x c x i = 600/2 x $15 x 0.20 = $9003. Total annual cost = $500 + $900 = $ 1,400.
III. Financial Inventory Performances
1. Inventory turnover = Annual CGS/Average Inventory $
Contoh: Bila Annual CGS 1 juta $, danAverage inventory = $ 500 ribu.Maka: Inventory turnover = 1 juta $/500 rb $ = 2.
2. Days of Supply = Inventory on hand/Average daily usage
Contoh: Perusahaan memiliki 9000 unit on hand, and annual usage = 48,000 unit. Ada 240 working days in the year.
Maka: Average daily usage = 48000/240 = 200 unit days of supply
IV. ABC AnalysisIV. ABC Analysis
• Divides on-hand inventory into 3 classesA 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
Classifying Items as ABCClassifying Items as ABC
020406080
100
0 20 40 60 80 1000
20406080
100
0 20 40 60 80 100% of Inventory Items% of Inventory Items
% Annual $ Usage% Annual $ UsageClass % $ Vol % Items
Classifying Items as ABCClassifying Items as ABC
020406080
100
0 20 40 60 80 1000
20406080
100
0 20 40 60 80 100% of Inventory Items% of Inventory Items
% Annual $ Usage% Annual $ Usage
AA
Class % $ Vol % ItemsA 80 15
Classifying Items as ABCClassifying Items as ABC
020406080
100
0 20 40 60 80 1000
20406080
100
0 20 40 60 80 100% of Inventory Items% of Inventory Items
% Annual $ Usage% Annual $ Usage
AA
BB
Class % $ Vol % ItemsA 80 15B 15 30
Classifying Items as ABCClassifying Items as ABC
020406080
100
0 20 40 60 80 1000
20406080
100
0 20 40 60 80 100% of Inventory Items% of Inventory Items
% Annual $ Usage% Annual $ Usage
AA
BB CC
Class % $ Vol % ItemsA 80 15B 15 30C 5 55
Contoh: ABC ClassificationContoh: ABC Classification
YouYou’’re a buyer for Auto Palace. Classify the re a buyer for Auto Palace. Classify the following items as A, B, or C.following items as A, B, or C.
Stock # Annual Volume (Units) Unit Cost206 26,000 $ 36105 200 600019 2,000 55144 20,000 4207 7,000 10
Note: Example is for illustration only; too few items.Note: Example is for illustration only; too few items.
ABC Classification SolutionABC Classification Solution
Stock # Vol. Cost $ Vol. % ABC206206 26,00026,000 $ 36$ 36105105 200200 600600019019 2,0002,000 5555144144 20,00020,000 44207207 7,0007,000 1010
TotalTotal
ABC Classification SolutionABC Classification Solution
Stock # Vol. Cost $ Vol. % ABC206206 26,00026,000 $ 36$ 36 $936,000$936,000105105 200200 600600019019 2,0002,000 5555144144 20,00020,000 44207207 7,0007,000 1010
TotalTotal
26,000 x 3626,000 x 36
ABC Classification SolutionABC Classification Solution
Stock # Vol. Cost $ Vol. % ABC206206 26,00026,000 $ 36$ 36 $936,000$936,000105105 200200 600600 120,000120,000019019 2,0002,000 5555 110,000110,000144144 20,00020,000 44 80,00080,000207207 7,0007,000 1010 70,00070,000
TotalTotal 1,316,0001,316,000
ABC Classification SolutionABC Classification Solution
Stock # Vol. Cost $ Vol. % ABC206206 26,00026,000 $ 36$ 36 $936,000$936,000 71.171.1105105 200200 600600 120,000120,000019019 2,0002,000 5555 110,000110,000144144 20,00020,000 44 80,00080,000207207 7,0007,000 1010 70,00070,000
TotalTotal 1,316,0001,316,000
936,0001,316,000936,000
1,316,000
ABC Classification SolutionABC Classification Solution
Stock # Vol. Cost $ Vol. % ABC206206 26,00026,000 $ 36$ 36 $936,000$936,000 71.171.1105105 200200 600600 120,000120,000 9.19.1019019 2,0002,000 5555 110,000110,000 8.48.4144144 20,00020,000 44 80,00080,000 6.16.1207207 7,0007,000 1010 70,00070,000 5.35.3
TotalTotal 1,316,0001,316,000 100.0100.0
ABC Classification SolutionABC Classification Solution
Stock # Vol. Cost $ Vol. % ABC206206 26,00026,000 $ 36$ 36 $936,000$936,000 71.171.1 AA105105 200200 600600 120,000120,000 9.19.1 AA019019 2,0002,000 5555 110,000110,000 8.48.4144144 20,00020,000 44 80,00080,000 6.16.1207207 7,0007,000 1010 70,00070,000 5.35.3
TotalTotal 1,316,0001,316,000 100.0100.0
ABC Classification SolutionABC Classification Solution
Stock # Vol. Cost $ Vol. % ABC206206 26,00026,000 $ 36$ 36 $936,000$936,000 71.171.1 AA105105 200200 600600 120,000120,000 9.19.1 AA019019 2,0002,000 5555 110,000110,000 8.48.4 BB144144 20,00020,000 44 80,00080,000 6.16.1 BB207207 7,0007,000 1010 70,00070,000 5.35.3
TotalTotal 1,316,0001,316,000 100.0100.0
ABC Classification SolutionABC Classification Solution
Stock # Vol. Cost $ Vol. % ABC206 26,000 $ 36 $936,000 71.1 A105 200 600 120,000 9.1 A019 2,000 55 110,000 8.4 B144 20,000 4 80,000 6.1 B207 7,000 10 70,000 5.3 C
Total 1,316,000 100.0