Safety Stock Management

1. SAFETY STOCK
MANAGEMENT
Adhe Priyambodo

2. 2
Adhe Priyambodo
Working history
Procurement Manager,
Shell
Direct Procurement Manager,
Frisian Flag Indonesia
RM Procurement Manager,
PZ Cussons Indonesia
Raw Material Manager,
Sepatu Bata, Tbk
Material Planner,
Henkel Indonesien
Education
Engineering Physics,
ITS Surabaya
Advance Diploma in Procurement & Supply,
Chartered Institute of Procurement and
Supply (CIPS), UK
About me
Procurement and supply chain management
professional with 16 years experience in various
industries of multinational companies.
Experienced trainer on the topics of
strategic procurement, negotiation,
and inventory management.
adhe.priyambodo@gmail.com
Adhe Priyambodo
http://priyambodo.net

3. Key topics
1. Definisi dan Fungsi Safety Stock
2. Menentukan Prioritas Safety Stock
3. Menentukan Jumlah Safety Stock
3

4. -1-
Definisi &
Fungsi
Safety Stock

5. Definisi & Fungsi Safety Stock
5
» Cycle stock adalah persediaan yang digunakan
untuk memenuhi permintaan reguler.
» Safety stock adalah persediaan yang
dicadangkan untuk mengantisipasi perubahan
dan harus ada sepanjang waktu.
» Safety stock berfungsi sebagai cadangan
terhadap fluktuasi permintaan (demand) dan
ketidakpastian waktu pasokan (lead time).

6. Bagan Persediaan: tanpa Safety Stock
6
Stock
Level
Time
Q
(Order Quantity)
L
(Lead time)
Q
(Order Quantity)
L
(Lead time)
R
(Reorder Level)
Q = ORDER QUANTITY
D = DEMAND
L = LEAD TIME
R = REORDER LEVEL
REORDER LEVEL = DEMAND x LEAD TIME

7. Menghitung Reorder Level - Contoh
7
DEMAND
= 90kg / 30hari
= 3 kg/hari
LEAD TIME
= 10 hari
REORDER LEVEL
= Demand x Lead Time
= 3 x 10
= 30 kg
Stock
Level
Time
90 kg
10 hari
Lead time
90 kg
10 hari
Lead time
30 hari 30 hari
Reorder
Level =
30 kg

8. Contoh – Menghitung Reorder Level
8
Jika diketahui:
» Lead time = 15 hari
» Demand = 1000 kg per bulan
Berapa Reoder Level-nya?

9. Contoh – Menghitung Reorder Level
9
DEMAND
= 1000kg / 30hari
= 33.33 kg/hari
LEAD TIME
= 15 hari
REORDER LEVEL
= Demand x Lead Time
= 33.33 x 15
= 500 kg
Stock
Level
Time
1000 kg
15 hari
Lead time
1000 kg
15 hari
Lead time
30 hari
Demand
30 hari
Demand
Reorder
Level =
500 kg

10. Bagan Persediaan: dengan Safety Stock
10
Stock
Level
Time
Q
(Order Quantity)
L
(Lead time)
Q
(Order Quantity)
L
(Lead time)
R
(Reorder Level)
Q = ORDER QUANTITY
D = DEMAND
L = LEAD TIME
R = REORDER LEVEL
S = SAFETY STOCK
S
(Safety Stock)
Q
(Order Quantity)
REORDER LEVEL = (DEMAND x LEAD TIME) + SAFETY STOCK

11. 11
STOCK-OUT
R
Q
0
SAFETY
STOCK
SAFETY STOCK
SAFETY STOCK
LEAD TIME
SAFETY STOCK = NEW REORDER LEVEL – NORMAL REORDER LEVEL
SAFETY STOCK = NEW REORDER LEVEL – NORMAL REORDER LEVEL

12. Scenario # 1
12
12
L = Lead Time
Reorder
Level
d = demand during Lead Time
0
NO Safety Stock
NO Safety Stock
Demand during lead time: FIXED
Lead time = FIXED
Demand during lead time: FIXED
Lead time = FIXED

13. Scenario # 2
13
L = Lead Time (fixed)
d = demand during Lead Time
Safety
Stock
d (normal)
d (higher demand)
Safety Stock to cover higher demand
Safety Stock to cover higher demand
Stockout
Reorder
Level
0
Demand during lead time: HIGHER
Lead time = FIXED
Demand during lead time: HIGHER
Lead time = FIXED

14. Scenario # 3
14
L = Lead Time (normal)
d = demand during Lead Time
Safety
Stock
Stockout
Safety Stock to cover longer lead time
Safety Stock to cover longer lead time
L = Lead Time (longer) d (firmed)
Reorder
Level
0
REORDER LEVEL = d X L
REORDER LEVEL = d X L
Demand during lead time: FIXED
Lead time = LONGER
Demand during lead time: FIXED
Lead time = LONGER

15. Scenario # 4
15
L = Lead Time (normal)
d = demand during Lead Time
Safety
Stock
Stockout
Safety Stock to cover longer lead time
and higher demand
Safety Stock to cover longer lead time
and higher demand
L = Lead Time (longer) d (normal)
d (higher demand)
Reorder
Level
0
Demand during lead time: HIGHER
Lead time = LONGER
Demand during lead time: HIGHER
Lead time = LONGER
REORDER LEVEL = d X L
REORDER LEVEL = d X L

16. Contoh – Skenario Safety Stock
16
Scenario
Demand
(units/day)
Lead time
(days)
Reorder Level
New Reorder
Level
Safety Stock
1
D and L
fixed 52 20 52 x 20 = 1040 0 0
2
D higher,
L fixed 57 20 52 x 20 = 1040 57 x 20 = 1140 1140 – 1040 = 100
3
D fixed,
L longer 52 24 52 x 20 = 1040 52 x 24 = 1248 1248 – 1040 = 208
4
D higher,
L longer 57 24 52 x 20 = 1040 57 x 24 = 1368 1368 – 1040 = 328
Semakin tinggi tingkat ketidakpastian Demand dan Lead time,
semakin tinggi Safety Stock yang dibutuhkan

17. -2-
Menentukan
Prioritas
Safety Stock

18. Menentukan Prioritas dengan ABC Analysis
18
» Class A
• 10 % of units
• 80 % of value
» Class B
• 30 % of units
• 15 % of value
» Class C
• 60 % of units
• 5 % of value 10 40 100
%
of
total
stock
value

19. Mengaplikasikan ABC Analysis
19
CLASS CONTROL RECORDS
STOCK
OPNAME
LOT SIZE
SAFETY
STOCK
A Tight Accurate Continue Small Small
B Medium Medium Sometime Medium Medium
C Simple Simple Rare Large High

20. Metode Alternatif
2
» Metode VEN
• Vital : zero shortage
• Essential : quick replenishment
• Non-essential : on order supply
» Metode FRS
• Fast moving : daily consumption
• Regular moving : above 7 days consumption
• Slow moving : above 30 days consumption

21. -3-
Menentukan
Jumlah
Safety Stock

22. Menentukan Jumlah Safety Stock
22
» Metode menghitung safety stock:
a) Cara Sederhana
1. Time Period Safety Stock
2. Maximum Safety Stock
b) Cara Statistik

23. a. Menghitung Safety Stock : Cara Sederhana
1. Time Period Safety Stock
Safety Stock = Average Usage x Safety Stock Period
Contoh:
- Average Usage : 40 unit / bulan
- Jumlah hari kerja : 20 hari / bulan → 5 hari/minggu
- Safety Stock Period : 2 minggu
Safety Stock = Average Usage x Safety Stock Period
= 40 unit/bulan x 2 minggu
= (40/20) unit/hari x (2*5) hari
= 20 unit
Total Stock = Average Usage + Safety Stock
= 40 unit + 20 unit
= 60 unit
• Cocok digunakan
untuk item fast
moving, essential,
dan value rendah sd.
sedang (kategori B
dan C)
• NilaiAverage Usage
disesuaikan dengan
jumlah hari kerja
setiap bulannya.
• Nilai Safety Stock
Period dievaluasi
secara periodik

24. a. Menghitung Safety Stock : Cara Sederhana
2. Maximum Safety Stock
Safety Stock = Max Demand x Max Lead Time
Contoh:
Dari data konsumsi dan suplay periode sebelumnya diperoleh:
- Average Demand : 20 unit / hari
- Average Lead time : 7 hari
- Maximum Demand : 25 unit / hari
- Maximum Lead time : 10 hari
Average Stock = Average Demand x Average LeadTime
= 20 unit/hari x 7 hari
= 140 unit
Total Stock = Maximum Demand x Maximum LeadTime
= 25 unit/hari x 10 hari
= 250 unit
Safety Stock =Total Stock – Average Stock
= 250 – 140
= 110
• Cocok digunakan
untuk item dengan
value rendah. Contoh:
item kategori C

25. b. Menghitung Safety Stock : Cara Statistik
» Variabel yang digunakan:
• Standar Deviasi demand (σ)
• Lead time (L).
• Safety Factor (z),
Safety Stock = Safety Factor x Standard Deviasi x √Lead-time
Asumsi:
1. Variasi demand besar
2. Variasi lead time kecil
ditentukan berdasarkan nilai Service Level yang diharapkan
» Direkomendasikan untuk item kategori “A” dan “B”.

26. Variability vs Service Level
26
z-score
0
-3.0 -2.0 -1.0 +1.0 +2.0 +3.0

27. Service Level & Safety Factor (z-score)

28. Tabel Safety Factor
28
Safety Factor
(z-score)
Formula Safety Factor (z)
di Excel :
=NORMSINV(xx%)

29. Contoh Safety Stock menggunakan Safety Factor (1)
29
Week # Demand (unit/week)
1 630
2 569
3 547
4 586
5 565
6 561
7 304
8 833
9 214
10 605
11 813
12 330
13 257
14 472
15 432
16 607
17 848
18 752
19 273
20 867
21 266
22 203
23 465
24 355
Diketahui:
• Demand per minggu (tabel)
• Lead time : 1 minggu
Hitunglah Safety Stock untuk Service Level
yang diharapkan sbb.:
• 50.00%
• 95.00%
• 99.00%
Solusi:
Standard Deviasi → =STDEV(data)
Standard Deviasi Demand = 210 unit

30. Contoh Safety Stock menggunakan Safety Factor (2)
30
Standard
Deviasi
Demand
(unit/week)
Desired
Service
Level
Safety Factor
(for std.dev)
Safety Stock
(unit)
210 50.00%
210 95.00%
210 99.00%
Dari tabel
Safety
Factor
Disepakati
oleh planner
& sales
Safety Factor
Dari data
historical
demand
Lead time = 1 week
Safety Stock = Safety Factor x Standard Deviasi x √Lead-time
0.00
1.65
2.33
0.00 x 210 x √1 = 0
1.65 x 210 x √1 = 346
2.33 x 210 x √1 = 489

31. 32
Thank you!
How to contact me:
adhe.priyambodo@gmail.com
Adhe Priyambodo
priyambodo.net