resume inventory

7/29/2019 Resume Inventory

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Kendali yang tepat untuk inventory (persediaan) merupakan hal yang penting untuk

mencapai kesuksesan dalam bisnis.

Masalah masalah mengenai inventory meliputi :

Basic Inventory Quantitiy discount Production lot size (produksi

kebanyakan)

Planned shortage

Periodic review Single period

Model Inventory seringkali digunakan untuk mengembangkan kebijakan inventory yang

optimal, meliputi :

Order Quantity , dengan simbol Q Reorder Point, dengan simbol RType of Costs in Inventory Models

Analisis Inventory dapat digunakan sebagai teknik pengendalian biaya. Kategori dariinventory models :

Holding (carrying costs) Order/ Setup costs Customer satisfaction costs Procurement/Manufacturing costsHolding Costs (Carrying costs/Biaya Penyimpanan):

Biaya ini tergantung pada ukuran pesanan (order size)

Cost of capital Storage space rental cost Costs of utilities Labor Insurance Security Theft (pencurian) and breakage (barang yg pecah) Deterioration (barang yg buruk)or Obsolescence(barang yg tidak dipakai lagi)Order/Setup Costs (Co)

Biaya ini tidak tergantung pada ukuran pesanan:

Order costs didatangkan ketika membeli barang dari supplier, meliputi : Telephone Order checking Labor Transportation

Setup costs didatangkan ketika memproduksi barang untuk dijual pada yanglain,meliputi : Cleaning machines Calibrating equipment Training staff

Customer Satisfaction Costs

Mengukur tingkat kepuasan pelanggan Pelanggan yang tidak puas disebabkan oleh :

Switch to the competition (lost sales). Wait until an order is supplied.

Ch = H * C

Ch = Annual holding cost per unitin inventory

H = Annual holding cost rateC = Unit cost of an item


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Ketika pelanggan menghendaki untuk menunggu, ada dua tipe biaya terjadi: Cb = Fixed administrative costs of an out of stock item ($/stockout unit). Cs = Annualized cost of a customer awaiting an out of stock item

($/stockout unit per year).

Procurement/Manufacturing Cost (C = Unit purchase/manufacturing cost) Menunjukkan biaya pembelian per unit (meliputi transportasi) dalam kasus pembelian Unit production cost in case of in-house manufacturing.Demand in Inventory Models [D = Demand rate (usually per year)]

Demand atau permintaan adalah komponen utama yang mempengaruhi kebijakaninventory.

Pola permintaan yang dirancang menentukan bagaimana masalah inventory dimodelkan. Macam macam pola permintaan adalah sbb:

Constant over time (deterministic inventory models) Changing but known over time (dynamic models)

Variable (randomly) over time (probabilistic models)

Inventory Classifications

Inventory dapat diklasifikasikan dalam beberapa cara:

Review Systems

Dua jenis review systems yang digunakan:

Continuous review systems. Sistem dimonitor terus menerus. Pesanan yang baru ditempatkan ketika inventory mencapai titik kritikal.

Periodic review systems. Posisi persediaan diinvestigasi berdasarkan basis reguler.

Order dilakukan hanya pada waktu ini.

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Demand terjadi pada tingkat yang diketahui dan masuk akal Item memiliki shelf life yang cukup panjang. Item dimonitor menggunakan continuous review system. Seluruh parameter biayanya selalu konstan (over an infinite time horizon). Pesanan yang lengkap diterima pada satu batch.

The EOQ Model Inventory profile

Lingkungan yang konstan dijelaskan dengan asumsi EOQ seperti observasi berikut:

Kebijakan EOQ yang optimal terdiri dari ukuran pesanan yang sama. Hasil observasi pada profil inventory adalah sbb

Management of items

with short shelf life and

long shelf life is very

different

Shelf life = waktu ygdibutuhkan untuk

Used typically byaccountants at

manufacturing firms.

Enables management to

track the productionrocess.

By Process By Importance By Shelf Life

Raw materials Perishable

Work in progress A, B, C Nonperishable

Finished goods

Items are classified by

their relative importance

in terms of the firms

capital needs.


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Cost Equation for the EOQ Model

The optimal order Size

Analisis sensitivitas

Cycle Time. Dimana T, menunjukkan waktu yang berlalu antara penempatan pesanan.

T = Q/D Jika cycle time lebih besar daripada shelf life, item nya kan menjadi buruk, dan ini

berarti model harus dimodifikasi.

Number of Orders per Year

N = D/Q

Example: The demand for a product is 1000 units per year.The order size is 250 units under an EOQ policy.

How many orders are placed per year? N = 1000/250 = 4 orders. How often orders need to be placed (what is the cycle time)?

T = 250/1000 = years. {Note: the four orders are equally spaced}.

Total AnnualInventory Costs = =

Total AnnualHolding Costs

Total Annualordering Costs

Total Annualprocurement Costs++

TC(Q) = (Q/2)Ch + (D/Q)Co + DC

QQ Q

Q* =

Ch

oDC2


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Lead Time and the Reorder Point

Dalam kenyataannya, lead time selalu ada,dan harus dihitung ketika memutuskanapakah akan melakukan pesanan.

Reorder point, R, adalah posisi inventory ketika melakukan order. R is calculated by : R = L D L and D must be expressed in the same time unit.

Lead Time and the Reorder Point Graphical demonstration: Short Lead Time

Safety stock

Safety stocks seperti penyangga, untuk mengatasi: Higher than average lead time demand. Longer than expected lead time.

With the inclusion of safety stock (SS), R is calculated by R = LD + SS Size of the safety stock tergantung pada service level yang diinginkan

Inventory Costs Including safety stock

8.3 Determining Safety Stock Levels

Bisnis memasukkan kebutuhan safety stock ketika menentukan reorder poin. Pendekatan yang memungkinkan untuk menentukan tingkat safety stock adalah

dengan menentukan tingkat layanan yang diinginkan.

Two Types of Service Level


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The cycle service level Probabilitas tidak mendatangkan stock out selama inventory cycle. Ini diterapkan ketika terdapat kemungkinan terjadi stockout, dan bukan

besarnya, hal ini penting bagi perusahaan. The unit service level

Persentase demands yang dapat terisi tanpa terkena delay. Ini diterapkan ketika persentase demand yang tidak memuaskan harus berada di

bawah control.

The Cycle Service Level Approach

Dalam beberapa kasus permintaan jangka pendek adalah variabel meskipunpermintaan jangka panjang diasumsikan konstan.

Oleh karena itu, stockout acara selama lead time dapat terjadi secara tak terdugapada setiap siklus.

stockouts terjadi hanya jika permintaan selama lead time lebih besar daripemesanan ulang.

Untuk menentukan titik pemesanan kembali perlu kita ketahui: Pembagian lead time permintaan.

Tingkat layanan yang diperlukan.

Dalam beberapa kasus, lead time demand mendekati terdistribusi normal.Untukkasus terdistribusi normal, reorder point dihitung dengan :

8.4 EOQ Models with Quantity Discounts

Quantity Discounts are Common Practice in Business Dengan menawarkan discounts,pembeli akanterdorong untuk meningkatkan jumlah

pembelian, sehingga dapat mengurangi holding cost dari penjual. Quantity discounts mencerminkan penghematan yang identik dengan order banyak.

Dengan quantity discounts penjual dapat menghargai pelanggan terbesar mereka tanpamelanggar aturan Robinson - Patman Act.

Quantity Discount Schedule This is a list ofper unit discounts and their corresponding purchase volumes. Normally, the price per unit declines as the order quantity increases. Jika order quantity = unit price changes is called a break point. There are two main discount plans:

All unit schedules : Harga dibayar untuk semua unit yang dibeli berdasarkan totalpembelian

Incremental schedules: Harga diskon berdasarkan hanya pada unit tambahan yang diorder melebihi break point.


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To determine the optimal order quantity, the total purchase cost must be includedTC(Q) = (Q/2)Ch + (D/Q)Co + DCi + ChSS

Ci : unit cost at the ith

pricing level.

8.5 Production Lot Size Model Assumptions Demand rate is constant. Production rate is larger than demand rate. The production lot is not received instantaneously (seketika) (at an infinite rate),

because production rate is finite(terbatas).

Hanya ada satu produk yang dijadwalkan. The rest of the EOQ assumptions stay in place. The optimal production lot size policy orders the same amount each time. This observation results in the inventory profile below:

Production Lot Size Model Total Variable Cost

The parameters of the total variable costs function are similar to those used in the EOQmodel.

Instead of ordering cost, we have here a fixed setup cost per production run (Co).In addition, we need to incorporate the annual production rate (P) in the model.

Production Lot Size Model Useful relationships

Cycle time T = Q / D. Length of a production run T1 = Q / P. Time when machines are not busy producing the product T2 = T - T1 = Q(1/D - 1/P). Average inventory = (Q/2)(1-D/P).

8.6 Planned Shortage Model

Kalo item yang dijual habis, pelanggan akan :


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Go somewhere else (lost sales)= kita jd kehilangan sales. Place their order and wait (backordering).= mw nunggu

Dalam model ini diasumsikan backordering case All the other EOQ assumptions are in place.

Planned Shortage Model the Total Variable Cost Equation The parameters of the total variable costs function are similar to those used in the EOQ

model.

In addition, we need to incorporate the shortage costs in the model. Backorder cost per unit per year (loss of goodwill cost) - Cs.

Reflects future reduction in profitability. Dapat diestimasi dari market surveys dan focus groups.

Backorder administrative cost per unit - Cb. Reflects additional work needed to take care of the backorder.

The Annual holding cost =Ch[T1/T](Average inventory) = Ch[T1/T] (Q-S)/2

The Annual shortage cost =Cb(number of backorders per year) + Cs(T2/T)(Average number of backorders).

To calculate the annual holding cost and shortage cost we need to find The proportion of time inventory is carried, (T1/T) The proportion of time demand is backordered, (T2/T).

Finding T1/ T and T2/ T

Annual holding cost:Ch[T1/T](Q-S)/2 = Ch[(Q-S) /Q](Q-S)/2

= Ch(Q-S)2/2Q

Annual shortage cost:Cb(Units in short per year) + Cs[T2/T](Average number of backorders) =

Cb(S)(D/Q) + CsS2/2Q

Planned Shortage Model The Total Variable Cost Equation The total annual variable cost equation


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The optimal solution to this problem is obtained under the followingconditions

Cs > 0 ; Cb < \/ 2CoCh / D

Planned Shortage Model The Optimal Inventory Policy

8.7 Review Systems Continuous Review

(R, Q) Policies EOQ, production lot size, and planned shortage models mengasumsikan:

inventory levels are continuously monitored Items are sold one at a time.

(R, Q) Policies The above models call for order point (R) order quantity (Q) inventory policies. Such policies can be implemented by

A point-of-sale computerized system. The two-bin system.

(R, M) policies Ketika hanya item tertentu saja yang terjual pada suatu waktu, reorder poin

mungkin akan hilang, dan situasi out of stock mungkin akan terjadi lagi lebih

sering

The order to level (R, M) policy may be implemented in this situation. (R,M) policies

The R, M policy replenishes(mengisi ulang) inventory up to a pre-determined level M.

Order Q = Q* + (R I) = (M SS) + (R I) each time the inventory falls tothe reorder point R or below.

(Order size may vary from one cycle to another).

Periodic Review Systems

Mungkin sulit atau mustahil untuk mengadopsi continuous review system, karena: The high price of a computerized system. Lack of space to adopt the two-bin system. Operations inefficiency when ordering different items from the same vendor

separately.

Periodic review system may be found more suitable for these situations. Dalam system ini, posisi inventory untuk tiap item akan diobservasi secara periodic. Orders for different items can be better coordinated periodically.

(T,M) Policies


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In a replenishment cycle policy (T, M), the inventory position is reviewedevery T time units.

An order is placed to bring the inventory level back up to a maximuminventory level M.

M is determined by Forecasting the number of units demanded during the review period T. Adding the desired safety stock to the forecasted demand.

Calculation of the replenishment level and order size

8.8 Single Period Inventory Model Assumptions

Demandnya stokastik dengan distribusi yang diketahui. Shelf life of the item is limited.(kadaluarsanya cepet) Inventory mudah terjual tidak lebih dari a single time period. Inventory is delivered only once during a time period. Pada akhir setiap periode, persediaan yang tidak terjual dibuang untuk menyelamatkan. Unsatisfied demand may result in shortage costs.The Expected Profit Function

Untuk menemukan optimal order quantity,we need to balance the expected cost of over-ordering and under ordering.

Expected Profit = (Profit when Demand=X)Prob(Demand=X)x

Expected profit: fungsi dari order size,random demand, & various costs.The Expected Profit Function : Developing an expression for EP(Q)

Notation

p = per unit selling price of the good.

c = per unit cost of the good.

s = per unit salvage value of unsold good.

K = fixed purchasing costs

Q = order quantity.

EP(Q) = Expected Profit if Q units are

ordered

Scenarios

Demand X is less than the order quantity (X < Q). Demand X is greater than or equal to the order quantity (X >Q.)

Scenario 1: Demand X less than() to the units stocked. Profit = pQ - g(X - Q) - cQ - K

The Optimal Solution : To maximize the expected profit order Q*

For the discrete demand case take the smallest value of Q* that satisfies the condition

P(D < Q*) > (p - c + g)/(p - s + g)

For the continuous demand case find the Q* that solves

F(Q*) = (p - c + g) /(p - s + g)

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