Konsep Dasar Persediaan Outline: 1. What is Inventory? 2. Key Factors in Inventory Management Decision 3. Financial Inventory Performances 4. 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.
Kinerja: Siklus Aliran Material
Input
other (6)
wait time (5)
move time (4)
queue time (3)
setup time (2)
run time (1)
Output
Cycle Time
(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.
Contoh Inventory • Stok material • Cadangan kapasitas Contoh:
II. Key Factors in Inventory Management Decision Key Factors in Inventory Management Decision: 1. Inventory management objectives 2. Flow of material 3. Pola supply dan demand 4. Inventory functions 5. Inventory costs.
2.1. Tujuan Inventory Management 1)
2) 3)
Maximum customer service: • Percentage of orders shipped on schedule • Percentage of line items shipped on schedule • Order-days out of stock • etc. Minimum inventory investments 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 material terdiri dari: 1. Raw materials 2. Work-in-process (WIP) 3. Finished goods 4. Distribution inventories 5. Maintenance, repair, operational supplies (MRO’s)
Contoh: Berdasarkan aliran material
Industri Komputer Persediaan (Inventory) Aliran Material
Raw Mat'l WIP Fin. Goods
.
Contoh: Berdasarkan Kuantitas & Nilai
Industri Otomotif A
Persediaan (Inventory)
Aliran Material
Kuantitas & Nilai
C Raw Mat'l WIP Fin. Goods
A Items B Items C Items
B
Contoh: Berdasarkan Jenis Permintaan
Persediaan (Inventory) Independent Aliran Material
Kuantitas & Nilai
Jenis Permintaan
Raw Mat'l WIP Fin. Goods
Item A Item B Item C
Independent Dependent Dependent
Contoh: MROs
Persediaan (Inventory)
Aliran Material
Raw Mat'l WIP Fin. Goods
Kuantitas & Nilai
Item A Item B Item C
Jenis Permintaan
Independent Dependent
Lain-lain
Mainten. Repair Operating
2.3. Pola Supply & Demand 1. Inventory tidak diperlukan, bila supply bisa memenuhi demand. 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 dapat didekati, bila produksi dapat dilakukan dengan line-flow
system.
3. Kondisi keseimbangan antara produksi dengan demand sulit dicapai, 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. 2. 3. 4. 5. 6.
Anticipatipation inventory Fluctuation inventory (safety stock) Lot size inventory Transportation inventory Hedge inventory MROs inventory
2.5. Total Cost of Inventory
Inventory cost components 1. 2. 3. 4. 5.
Item cost: price of item, incl. transportation cost, custom and duties, and insurance (landed costs) Carrying cost: incl. capital costs, storage costs, risk costs (obsolescence, damage, pilferage, deterioration) Ordering cost: incl. production control costs, setup and teardown costs, lost capacity cost, purchase order cost. Stockout cost: could be backorder costs, losts sales, and possibly lost customers. 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. 2.
3.
Biaya pemesanan per tahun (annual ordering cost) = Frekuensi pemesanan x Biaya per pemesanan = A/Q x S 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 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 unit S = $30 i = 0.20 c = $15 Q = 600 unit. Jadi: 1. Annual ordering cost = A/Q x S = 10000/600 x $30= $500 2. Annual carrying cost = Q/2 x c x i = 600/2 x $15 x 0.20 = $900 3. Total annual cost = $500 + $900 = $ 1,400.
III. Financial Inventory Performances 1. Inventory turnover = Annual CGS/Average Inventory $ Contoh: Bila Annual CGS 1 juta $, dan Average 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 Analysis • 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
Classifying Items as ABC % Annual $ Usage Class % $ Vol % Items
100 80 60 40 20 0 0
20
40
60
80
% of Inventory Items
100
Classifying Items as ABC % Annual $ Usage
100 80
Class % $ Vol % Items A 80 15
60
A
40 20 0 0
20
40
60
80
% of Inventory Items
100
Classifying Items as ABC
% 100 Annual $ Usage
80
Class % $ Vol % Items A 80 15 B 15 30
60
A
40 20
B
0 0
20
40
60
80
% of Inventory Items
100
Classifying Items as ABC % Annual $ Usage Class % $ Vol % Items A 80 15 B 15 30 C 5 55
100 80 60
A
40 20
B
C
0 0
20
40
60
80
% of Inventory Items
100
Contoh: ABC Classification You’re a buyer for Auto Palace. Classify the following items as A, B, or C. Stock #
Annual Volume (Units)
Unit Cost
206 105 019 144 207
26,000 200 2,000 20,000 7,000
$ 36 600 55 4 10
Note: Example is for illustration only; too few items.
ABC Classification Solution
Stock #
Vol.
206 105 019 144 207
26,000 200 2,000 20,000 7,000
Total
Cost $ 36 600 55 4 10
$ Vol.
%
ABC
ABC Classification Solution
Stock #
Vol.
206 105 019 144 207
26,000 200 2,000 20,000 7,000
Total
Cost
$ Vol.
%
ABC
$ 36 $936,000 600 55 26,000 26,000 xx 36 36 4 10
ABC Classification Solution
Stock #
Vol.
206 105 019 144 207
26,000 200 2,000 20,000 7,000
Total
Cost
$ Vol.
$ 36 $936,000 600 120,000 55 110,000 4 80,000 10 70,000 1,316,000
%
ABC
ABC Classification Solution
Stock #
Vol.
206 105 019 144 207
26,000 200 2,000 20,000 7,000
Total
Cost
$ Vol.
$ 36 $936,000 600 120,000 55 110,000 4 80,000 10 70,000 1,316,000
%
ABC
71.1
936,000 936,000 1,316,000 1,316,000
ABC Classification Solution
Stock #
Vol.
206 105 019 144 207
26,000 200 2,000 20,000 7,000
Total
Cost
$ Vol.
%
$ 36 $936,000 71.1 600 120,000 9.1 55 110,000 8.4 4 80,000 6.1 10 70,000 5.3 1,316,000 100.0
ABC
ABC Classification Solution
Stock #
Vol.
206 105 019 144 207
26,000 200 2,000 20,000 7,000
Total
Cost
$ Vol.
%
ABC
$ 36 $936,000 71.1 A 600 120,000 9.1 A 55 110,000 8.4 4 80,000 6.1 10 70,000 5.3 1,316,000 100.0
ABC Classification Solution
Stock #
Vol.
206 105 019 144 207
26,000 200 2,000 20,000 7,000
Total
Cost
$ Vol.
%
$ 36 $936,000 71.1 600 120,000 9.1 55 110,000 8.4 4 80,000 6.1 10 70,000 5.3 1,316,000 100.0
ABC A A B B
ABC Classification Solution
Stock #
Vol.
206 105 019 144 207
26,000 200 2,000 20,000 7,000
Total
Cost
$ Vol.
$ 36 $936,000 600 120,000 55 110,000 4 80,000 10 70,000
% 71.1 9.1 8.4 6.1 5.3
1,316,000 100.0
ABC A A B B C
