Inventory Management MODEL PERSEDIAAN n TUJUAN Mengetahui modelmodel

Inventory Management : MODEL PERSEDIAAN n TUJUAN Mengetahui model-model pengelolaan persediaan

MODEL PERSEDIAAN n Tujuan n Ukuran persediaan berhubungan dengan ukuran pesanan, frekuensi pesan n 1. 2. menentukan ukuran persediaan Item dengan permintaan atau kebutuhan yang relatif stabil dalam jangka panjang, ukuran pesanan berdampak pada frekuensi pesan rata-rata persediaan.

TRADE OFF Menentukan ukuran pesanan Makin besar ukuran pesanan : n frekuensi pesan lebih kecil n rata-rata persediaan besar n biaya pesan kecil n biaya simpan besar Sebaliknya untuk ukuran pesan kecil.

Model Pengelolaan Persediaan Item dengan pasokan relatif stabil : n dapat menggunakan model EOQ untuk menentukan ukuran pesanan yang ekonomis. n Ketidakpastian diakomodasi dengan menentukan reorder point dan safety stock

Model Persediaan Item bersifat musiman : n Penting mempertimbangkan trade off antara biaya kelebihan dan kekurangan persediaan. n Resiko kelebihan dapat dikurangi dengan (1) mengurangi harga jual di musim jual, (2) mengurangi lead time sehingga responsif terhadap pasar

VENDOR MANAGED INVENTORY Vendor Managed Inventory (VMI) n Model pengelolaan persediaan dimana keputusan waktu dan ukuran pengiriman ditentukan supplier. n Pembeli memberikan informasi yang up to date tentang kondisi persediaan dan kebutuhan

• Biaya pesan dan simpan • Demand is known and constant D units per time. • No stock-outs! • Leadtime=0! Costs: K: order cost c: Unit variable cost h: inventory holding cost Q : order size (decision var. ) Inventory Q -D T Time

Deriving EOQ n n Total cost at every cycle: C(Q)=K+c. Q Average inventory holding cost in a cycle: Q/2 Cycle time T =Q/D G(Q)=(K+c. Q)/T + h. Q/2 = KD/Q + Dc + h. Q/2

EOQ: Costs G(Q) Ordering Cost Holding cost Total cost G(Q) Q Q*

Stochastic Demand n Usually the demand has a variable component D=Dconstant+Dvariable

Inventory Policy

Continuous review, reorder policy – demand rate L – replenishment lead time S: inventory reorder level; Q: reorder size inventory +Q +Q S d. L= L Safety stock: S-d. L L place order arrives time

Periodic Review, Order-upto Policy Define: Inventory Position = Quantity on hand + Quantity on order S - Base stock level/Order-up-to Point R- Review period L- Replenishment lead time - Demand per unit time Q - Order quantity ss - Safety stock Ordering Rule: Place an order every R periods so as to bring your inventory position to the Base Stock Level, S.

Periodic review with no demand variability Inventory position On-hand inventory Inventory Level (R+L) R L 0 R L 2 R R+L 3 R 2 R+L 4 R 3 R+L time

Periodic review with no demand variability Order Quantity, Q = R Average Cycle stock = Q/2 = R / 2 Pipeline stock = L Order-up-to point, S = (R+L)

Periodic review with variable demand Order-up-to point (S) = (R+L) + Safety Stock (ss) Average Order Quantity (Q) = R Average Pipeline stock = L Average Cycle stock = Q/2 = R / 2 Average Safety Stock = ss = ?

Determination of the Safety Stock Inventory Level Inventory position On-hand inventory R+ L+ss R+ss L+ss ss 0 R L 2 R R+L 3 R 2 R+L 4 R 3 R+L time

Inventory Level Freq Inventory On-hand X Place order Lead Time Receive order Time

Inventory Level Freq Inventory On-hand X + =S Safety Stock (SS) Place order Lead Time Receive order Time

Given R, L, , and : Safety Stock (ss) = Choose z such that: Fill Rate (p) = Expected standard loss (shortage) Demand variability over R + L units time Average demand filled period So: where L(z) is the standard loss function Base Stock Level (S) =

Example #1 Given: R = 2 weeks L= 1 week = 150 units per week = 10 units per week Target fill rate, p=99% Solve: so from table, z = 0. 58 Safety stock = Base stock level =

Example #2 Given: R = 2 weeks L= 1 week = 150 units per week = 10 units per week Target fill rate, p=95% Solve: so from table, z = -0. 73 Safety stock = Base stock level =