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Manajemen Persediaan

Bab 11-12. Manajemen Persediaan. Persediaan. Stok barang yang dimiliki untuk memenuhi kebutuhan masa depan Manajemen persediaan menjawab dua pertanyaan Berapa banyak pesanan waktu pemesanan. Tipe Persediaan. Bahan baku Bagian yang dibeli dan persediaan Tenaga Kerja

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Manajemen Persediaan

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  1. Bab11-12 ManajemenPersediaan

  2. Persediaan • Stokbarang yang dimilikiuntukmemenuhikebutuhanmasadepan • Manajemenpersediaanmenjawabduapertanyaan • Berapabanyakpesanan • waktupemesanan

  3. Tipe Persediaan • Bahanbaku • Bagian yang dibelidanpersediaan • TenagaKerja • Produkdalamproses • bagiankomponen • modal kerja • Alat , mesin , danperalatan

  4. Alasan menggunakan inventori • Memenuhipermintaantakterduga • permintaanmusimanatausiklus yang smooth • Memenuhivariasipermintaanpelanggan • Mengambilkeuntungandaridiskonharga • Melindungnilaiterhadapkenaikanharga • kuantitasdiskon

  5. Dua bentuk permintaan • Dependent • Item yang digunakanuntukmenghasilkanprodukakhir • Independent • Item yang dimintaolehpelangganeksternal

  6. Inventory Costs • Carrying Cost • Biayapengadaan item dalampersediaan • Ordering Cost • Biayapengisianpersediaankekurangan • Shortage Cost • Kerugiansementaraataupermanendaripenjualanketikapermintaantidakdapatdipenuhi

  7. Inventory Control Systems • Sistemkontinyu ( fixed -order - kuantitas ) • Jumlahkonstandiorderketikapersediaanmenurunke level yang telahditentukan • Sistemperiodik ( - periodewaktu yang tetap ) • Pesananditempatkanuntukjumlahvariabelsetelahberlalunyawaktu yang tetap

  8. PERCENTAGE PERCENTAGE CLASS OF UNITS OF DOLLARS A 5 - 15 70 - 80 B 30 15 C 50 - 60 5 - 10 ABC Classification System • Volume permintaandannilaibarangbervariasiMengklasifikasikanpersediaanmenjadi 3 kategori , biasanyaatasdasarnilaidolaruntukperusahaan

  9. PART UNIT COST ANNUAL USAGE 1 $ 60 90 2 350 40 3 30 130 4 80 60 5 30 100 6 20 180 7 10 170 8 320 50 9 510 60 10 20 120 ABC Classification Example 10.1

  10. TOTAL % OF TOTAL % OF TOTAL PART VALUE VALUE QUANTITY % CUMMULATIVE PART UNIT COST ANNUAL USAGE 9 $30,600 35.9 6.0 6.0 8 16,000 18.7 5.0 11.0 2 14,000 16.4 4.0 15.0 1 5,400 6.3 9.0 24.0 4 4,800 5.6 6.0 30.0 3 3,900 4.6 10.0 40.0 6 3,600 4.2 18.0 58.0 5 3,000 3.5 13.0 71.0 10 2,400 2.8 12.0 83.0 7 1,700 2.0 17.0 100.0 $85,400 1 $ 60 90 2 350 40 3 30 130 4 80 60 5 30 100 6 20 180 7 10 170 8 320 50 9 510 60 10 20 120 ABC Classification Example 10.1

  11. TOTAL % OF TOTAL % OF TOTAL PART VALUE VALUE QUANTITY % CUMMULATIVE PART UNIT COST ANNUAL USAGE 9 $30,600 35.9 6.0 6.0 8 16,000 18.7 5.0 11.0 2 14,000 16.4 4.0 15.0 1 5,400 6.3 9.0 24.0 4 4,800 5.6 6.0 30.0 3 3,900 4.6 10.0 40.0 6 3,600 4.2 18.0 58.0 5 3,000 3.5 13.0 71.0 10 2,400 2.8 12.0 83.0 7 1,700 2.0 17.0 100.0 $85,400 1 $ 60 90 2 350 40 3 30 130 4 80 60 5 30 100 6 20 180 7 10 170 8 320 50 9 510 60 10 20 120 ABC Classification A B C Example 10.1

  12. TOTAL % OF TOTAL % OF TOTAL PART VALUE VALUE QUANTITY % CUMMULATIVE PART UNIT COST ANNUAL USAGE 9 $30,600 35.9 6.0 6.0 8 16,000 18.7 5.0 11.0 2 14,000 16.4 4.0 15.0 1 5,400 6.3 9.0 24.0 4 4,800 5.6 6.0 30.0 3 3,900 4.6 10.0 40.0 6 3,600 4.2 18.0 58.0 5 3,000 3.5 13.0 71.0 10 2,400 2.8 12.0 83.0 7 1,700 2.0 17.0 100.0 $85,400 A 1 $ 60 90 2 350 40 3 30 130 4 80 60 5 30 100 6 20 180 7 10 170 8 320 50 9 510 60 10 20 120 % OF TOTAL % OF TOTAL CLASS ITEMS VALUE QUANTITY B A 9, 8, 2 71.0 15.0 B 1, 4, 3 16.5 25.0 C 6, 5, 10, 7 12.5 60.0 C ABC Classification Example 10.1

  13. 100 – 80 – 60 – 40 – 20 – 0 – C B A % of Value | | | | | | 0 20 40 60 80 100 % of Quantity ABC Classification

  14. Asumsi dasar model EOQ • Permintaandiketahuidenganpastidankonstandariwaktukewaktu • Tidakadakekurangan yang diperbolehkan • Lead time untukmenerimaperintahkonstan • Jumlahpesananditerimasekaligus

  15. Order quantity, Q Demand rate Inventory Level Reorder point, R 0 Time Lead time Lead time Order placed Order receipt Order placed Order receipt The Inventory Order Cycle Figure 10.1

  16. Annual ordering cost = CoD Q CoD Q Annual carrying cost = CcQ 2 CcQ 2 Total cost = + EOQ Cost Model Co - cost of placing order D - annual demand Cc - annual per-unit carrying cost Q - order quantity

  17. Deriving Qopt Proving equality of costs at optimal point CoD Q CcQ 2 TC = + CoD Q2 Annual ordering cost = Cc 2 TC Q = + = CoD Q CoD Q C0D Q2 Cc 2 Annual carrying cost = 0 = + 2CoD Cc CoD Q CcQ 2 CcQ 2 CcQ 2 Q2 = 2CoD Cc Qopt = 2CoD Cc Total cost = + Qopt = EOQ Cost Model Co - cost of placing order D - annual demand Cc - annual per-unit carrying cost Q - order quantity

  18. CoD Q CcQ 2 TCmin = + Qopt = 2(150)(10,000) (0.75) (150)(10,000) 2,000 (0.75)(2,000) 2 Qopt = TCmin = + 2CoD Cc Qopt = 2,000 yards TCmin = $750 + $750 = $1,500 EOQ Example Cc = $0.75 per yard Co = $150 D = 10,000 yards Orders per year = D/Qopt = 10,000/2,000 = 5 orders/year Order cycle time = 311 days/(D/Qopt) = 311/5 = 62.2 store days Example 10.2

  19. Inventory level Maximum inventory level Q(1-d/p) Average inventory level Q 2 (1-d/p) 0 Time EOQ with Noninstantaneous Receipt Figure 10.3

  20. Inventory level Maximum inventory level Q(1-d/p) Average inventory level Q 2 (1-d/p) 0 Begin order receipt End order receipt Time Order receipt period EOQ with Noninstantaneous Receipt Figure 10.3

  21. 2CoD Cc1 - Q p Maximum inventory level = Q - d = Q 1 - Qopt = d p d p d p CoD Q CcQ 2 Q 2 d p Average inventory level = 1 - TC = + 1 - EOQ with Noninstantaneous Receipt p = production rate d = demand rate

  22. 2CoD Cc1 - Qopt = = = 2,256.8 yards d p Q p 2,256.8 150 CoD Q CcQ 2 d p 32.2 150 TC = + 1 - = $1,329 2(150)(10,000) 0.75 1 - Production run = = = 15.05 days per order Production Quantity Cc = $0.75 per yard Co = $150 D = 10,000 yards d = 10,000/311 = 32.2 yards per day p = 150 yards per day Example 10.3

  23. 2CoD Cc1 - Qopt = = = 2,256.8 yards d p D Q 10,000 2,256.8 Number of production runs = = = 4.43 runs/year Q p 2,256.8 150 CoD Q CcQ 2 d p 32.2 150 TC = + 1 - = $1,329 2(150)(10,000) 0.75 1 - d p 32.2 150 Maximum inventory level = Q 1 - = 2,256.8 1 - = 1,772 yards Production run = = = 15.05 days per order Production Quantity Cc = $0.75 per yard Co = $150 D = 10,000 yards d = 10,000/311 = 32.2 yards per day p = 150 yards per day Example 10.3

  24. CoD Q CcQ 2 TC = + + PD where P = per unit price of the item D = annual demand Quantity Discounts • Price per unit decreases as order quantity increases

  25. CoD Q CcQ 2 TC = + + PD ORDER SIZE PRICE 0 - 99 $10 100 - 199 8 (d1) 200+ 6 (d2) where P = per unit price of the item D = annual demand Quantity Discounts • Price per unit decreases as order quantity increases

  26. TC = ($10 ) TC (d1 = $8 ) TC (d2 = $6 ) Inventory cost ($) Carrying cost Ordering cost Q(d1 ) = 100 Qopt Q(d2 ) = 200 Quantity Discount Model Figure 10.4

  27. TC = ($10 ) TC (d1 = $8 ) TC (d2 = $6 ) Inventory cost ($) Carrying cost Ordering cost Q(d1 ) = 100 Qopt Q(d2 ) = 200 Quantity Discount Model Figure 10.4

  28. QUANTITY PRICE 1 - 49 $1,400 50 - 89 1,100 90+ 900 2CoD Cc 2(2500)(200) 190 Qopt = = = 72.5 PCs For Q = 72.5 CoD Qopt CcQopt 2 For Q = 90 CcQ 2 CoD Q TC = + + PD = $233,784 TC = + + PD = $194,105 Quantity Discount Co = $2,500 Cc = $190 per computer D = 200 Example 10.4

  29. When to Order Reorder Point is the level of inventory at which a new order is placed R = dL where d = demand rate per period L = lead time

  30. Reorder Point Example Demand = 10,000 yards/year Store open 311 days/year Daily demand = 10,000 / 311 = 32.154 yards/day Lead time = L = 10 days R = dL = (32.154)(10) = 321.54 yards Example 10.5

  31. Safety Stocks • Safety stock • buffer added to on hand inventory during lead time • Stockout • an inventory shortage • Service level • probability that the inventory available during lead time will meet demand

  32. Q Inventory level Reorder point, R 0 Time Variable Demand with a Reorder Point Figure 10.5

  33. Q Inventory level Reorder point, R 0 LT LT Time Variable Demand with a Reorder Point Figure 10.5

  34. Q Inventory level Reorder point, R Safety Stock 0 LT LT Time Reorder Point with a Safety Stock Figure 10.6

  35. R = dL + zd L where d = average daily demand L = lead time d = the standard deviation of daily demand z = number of standard deviations corresponding to the service level probability zd L = safety stock Reorder Point With Variable Demand

  36. Probability of meeting demand during lead time = service level Probability of a stockout Safety stock zd L dL Demand R Reorder Point for a Service Level Figure 10.7

  37. d = 30 yards per day L = 10 days d = 5 yards per day R = dL + zd L = 30(10) + (1.65)(5)( 10) = 326.1 yards Safety stock = zd L = (1.65)(5)( 10) = 26.1 yards Reorder Point for Variable Demand The carpet store wants a reorder point with a 95% service level and a 5% stockout probability For a 95% service level, z = 1.65 Example 10.6

  38. Q = d(tb + L) + zdtb + L - I where d = average demand rate tb = the fixed time between orders L = lead time sd = standard deviation of demand zdtb + L = safety stock I = inventory level Order Quantity for a Periodic Inventory System

  39. d = 6 bottles per day sd = 1.2 bottles tb = 60 days L = 5 days I = 8 bottles z = 1.65 (for a 95% service level) Q = d(tb + L) + zdtb + L - I = (6)(60 + 5) + (1.65)(1.2) 60 + 5 - 8 = 397.96 bottles Fixed-Period Model with Variable Demand

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