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

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1 Inventory Management : MODE PERSEDIAAN TUJUAN Mengetahui model-model pengelolaan persediaan

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

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

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

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

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

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

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

9 EOQ: Costs G(Q) Ordering Cost Total cost Q * Holding cost G(Q) hq 2 KD Q Q

10 Stochastic Demand Usually the demand has a variable component D=D constant +D variable

11 Inventory Policy Continuous Periodic Type A (s,s) (R,s,S) Type B (s,q ) (R,S)

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

13 Periodic Review, Orderup-to Policy Define: Inventory Position = Quantity + Quantity on hand on order S - Base stock level/order-up-to Point R- Review period - Replenishment lead time m - 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 evel, S.

14 Periodic review with no demand variability Inventory evel Inventory position On-hand inventory m(r+) mr m 0 R 2R 3R 4R R+ 2R+ 3R+ time

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

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

17 Determination of the Safety Stock Inventory evel Inventory position On-hand inventory mr+m+ss mr+ss m+ss ss 0 R 2R 3R 4R R+ 2R+ 3R+ time

18 Inventory evel Inventory On-hand Freq X Place order ead Time Receive order Time

19 Inventory evel Inventory On-hand Freq X m ( R ) + SS z R = S Place order ead Time Safety Stock (SS) Receive order Time

20 Given R,, m, and : Safety Stock (ss) = Choose z such that: Fill Rate (p) = 1 ( z) z R R mr Expected standard loss (shortage) Demand variability over R + units time Average demand filled per period So: ( z) 1 p mr R where (z) is the standard loss function Base Stock evel (S) = m R m z R

21 Example #1 Given: Solve: R = 2 weeks = 1 week m = 150 units per week = 10 units per week Target fill rate, p=99% z 1 p mr R so from table, z = 0.58 Safety stock = ss z R Base stock level = S mr m z R

22 Example #2 Given: Solve: R = 2 weeks = 1 week m = 150 units per week = 10 units per week Target fill rate, p=95% z 1 p mr R so from table, z = Safety stock = ss z R Base stock level = S m mr z R