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 Produk dalam proses bagian komponen modal kerja Alat , mesin , dan peralatan

Alasan menggunakan inventori Memenuhi permintaan tak terduga permintaan musiman atau siklus yang smooth Memenuhi variasi permintaan pelanggan Mengambil keuntungan dari diskon harga Melindung nilai terhadap kenaikan harga kuantitas diskon

Dua bentuk permintaan Dependent Independent Item yang digunakan untuk menghasilkan produk akhir Independent Item yang diminta oleh pelanggan eksternal

Inventory Costs Carrying Cost Biaya pengadaan item dalam persediaan Ordering Cost Biaya pengisian persediaan kekurangan Shortage Cost Kerugian sementara atau permanen dari penjualan ketika permintaan tidak dapat dipenuhi

Inventory Control Systems Sistem kontinyu ( fixed -order - kuantitas ) Jumlah konstan diorder ketika persediaan menurun ke level yang telah ditentukan Sistem periodik ( - periode waktu yang tetap ) Pesanan ditempatkan untuk jumlah variabel setelah berlalunya waktu yang tetap

ABC Classification System Volume permintaan dan nilai barang bervariasi Mengklasifikasikan persediaan menjadi 3 kategori , biasanya atas dasar nilai dolar untuk perusahaan PERCENTAGE PERCENTAGE CLASS OF UNITS OF DOLLARS A 5 - 15 70 - 80 B 30 15 C 50 - 60 5 - 10

ABC Classification PART UNIT COST ANNUAL USAGE 1 $ 60 90 2 350 40 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 PART UNIT COST ANNUAL USAGE Example 10.1

ABC Classification PART UNIT COST ANNUAL USAGE 1 $ 60 90 2 350 40 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 PART UNIT COST ANNUAL USAGE TOTAL % OF TOTAL % OF TOTAL PART VALUE VALUE QUANTITY % CUMMULATIVE 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 Example 10.1

ABC Classification A B C PART UNIT COST ANNUAL USAGE 1 $ 60 90 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 PART UNIT COST ANNUAL USAGE TOTAL % OF TOTAL % OF TOTAL PART VALUE VALUE QUANTITY % CUMMULATIVE 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 B C Example 10.1

ABC Classification A B C PART UNIT COST ANNUAL USAGE 1 $ 60 90 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 PART UNIT COST ANNUAL USAGE TOTAL % OF TOTAL % OF TOTAL PART VALUE VALUE QUANTITY % CUMMULATIVE 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 B C % OF TOTAL % OF TOTAL CLASS ITEMS VALUE QUANTITY 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 Example 10.1

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

Asumsi dasar model EOQ Permintaan diketahui dengan pasti dan konstan dari waktu ke waktu Tidak ada kekurangan yang diperbolehkan Lead time untuk menerima perintah konstan Jumlah pesanan diterima sekaligus

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

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

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

EOQ Example Cc = $0.75 per yard Co = $150 D = 10,000 yards Qopt = 2CoD 2(150)(10,000) (0.75) Qopt = 2,000 yards TCmin = + CoD Q CcQ 2 TCmin = + (150)(10,000) 2,000 (0.75)(2,000) TCmin = $750 + $750 = $1,500 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

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

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

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

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 Qopt = = = 2,256.8 yards 2CoD Cc 1 - d p 2(150)(10,000) 0.75 1 - 32.2 150 TC = + 1 - = $1,329 d p CoD Q CcQ 2 Production run = = = 15.05 days per order Q p 2,256.8 150 Example 10.3

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

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

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

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

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

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

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

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

Safety Stocks Safety stock Stockout Service level 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

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

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

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

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

Reorder Point for Variable Demand The carpet store wants a reorder point with a 95% service level and a 5% stockout probability d = 30 yards per day L = 10 days d = 5 yards per day For a 95% service level, z = 1.65 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 Example 10.6

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

Fixed-Period Model with Variable Demand 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d tb + L - I = (6)(60 + 5) + (1.65)(1.2) 60 + 5 - 8 = 397.96 bottles