Continuous Review Systems in Inventory Management

Talia GruberMarriott SchoolNovember 2011

Agenda

• Continuous review systems defined

• Inventory positions and reorder points

• Reorder points with constant demand and lead time

• Reorder points with variable demand and constant lead time

• Reorder points with variable demand and lead time

• Two-Bin System

What are Continuous Review Systems?

• Systems that determine whether it is time to reorder a product by tracking the remaining inventory levels of the product each time a withdrawal is made.

Alternate Names• The Continuous Review System may also be called:

– Reorder Point (ROP) System

– Fixed Order-quantity System

– Quantity and Reorder Point (QR) System

– Q System

Inventory Position

Inventory Position (IP) measures the level of the current product’s ability to fulfill future orders.

IP = OH + SR - BO

Reorder Point

A predetermined minimum level that triggers the order of a fixed quantity of the product.

Selecting the Reorder Point with Constant Demand and Lead Time

On-hand

Inventory

Time

Order

Received

Order Placed

Inventory Position

R

L L L

Practice Problem with Constant Demand and Lead Time

Demand for water bottles from a particular concession stand is always 30 per day and the lead time is always 2 days. The on hand inventory is currently 15 bottles, and there is is an incoming order of 50 water bottles. If there are currently no backorders, what is the inventory position? Should a new order be placed?

Solution

R = Total demand during lead time = 30*2 = 60 bottles

IP = OH + SR – BO

= 15 + 50 – 0 = 65 bottles

65 > 60 so a new order is NOT needed

Selecting Reorder Point with Variable Demand and Constant

Lead Time• Demand is not usually constant and

predictable

• To guard against stock-outs, reorders that include safety stock should be selected

Reorder Point = Avg. demand during lead time + Safety stock

Selecting Reorder Point with Variable Demand and Constant

Lead Time

On-hand

Inventory

TimeOrder Placed

Inventory Position

R

Order Received

Selecting Reorder Point with Variable Demand and Constant

Lead Time

Three steps to selecting a reorder point:

1. Choose an appropriate service-level policy

2. Determine the demand during lead time probability distribution

3. Determine the safety stock and reorder point levels

Step 1: Service Level Policy

Service Level: The desired probability of not running out of stock during an order cycle

An 80% service cycle = An 80% chance that the stock will not run out during the lead time.

Step 2: Demand During Lead Time Distribution

• In order to calculate the specific safety stock level, you must determine the demand during lead time distribution by knowing the:– Mean– Standard Deviation

Step 2 Continued• Determine the demand during the lead time

distribution by making the following assumptions:– Average demand is known– Standard deviation of demand is known– Probability distributions of demand are

independent and identical over each time period.

Step 2 Formulas

Average demand during lead time:

d + d + d + … = dL

Variance of demand during lead time:

σ2d+ σ2

d + σ2d + … = σ2

dL

Standard deviation of demand during lead time:

σdLT = √( σ2dL) = σd√ (L)

Step 2 Calculations

+ + =

Demand week 1 Demand week 2 Demand week 3 Demand for 3-week

lead time

σd = 10 σd = 10 σd = 10

50 50 50 150

σdLT = 17.32

Step 3: Safety Stock and Reorder Point

Average

demand

during lead

time

50%

Service Level = 50% Service Level = 90%

Average

demand

during lead

time

90%

=

Step 3 Continued: Formulas

Safety Stock = zσdLT

Z = The number of standard deviations needed to achieve the service level

σdLT = Standard deviation of demand during lead time

Reorder Point = dL + safety stock

Practice Problem with Variable Demand and Constant Lead Time

The average weekly demand for water bottles from a particular concession stand is 80 bottles per week, and the constant lead time is 3 weeks. There is a standard deviation of 10 bottles. If a 95% service level is desired, what would the safety stock and reorder point be?

Solutiond = 80 bottles

σd = 10 bottles

L = 3 weeks

σdLT = σd√ (L) = 10√(3) = 17.32

z = 1.64

Safety stock = zσdLT = (1.64)(17.32) = 28.40 or 28 bottles

Reorder Point = dL + safety stock = (80)(3) + 28 = 268 bottles

Selecting Reorder Point with Variable Demand and Lead Time

Two assumptions:

– Demand and lead time are measured in the same time units (i.e. days, weeks, etc.)

– Demand and lead time are independent of one another

Formulas for Variable Demand and Variable Lead Time

Safety stock = zσdLT

R = (Avg. weekly demand x Avg. lead time in weeks) + safety stock

= dL + safety stock

Practice Problem with Variable Demand and Lead Time

The average weekly demand for water bottles from a particular concession stand is 50 bottles per week, with a standard deviation of 10 bottles. The average lead time is 2 weeks, with a standard deviation of 1 week. If an 80% service level is desired, what would the reorder point be?

Solutiond = 50 bottles

σd = 10 bottles

L = 2 weeks

σLT = 1 week

σdLT = √(Lσ2d + d2σ2

LT) = √[(2)(10)2 + (50)2(1)2] = 51.96 bottles

z = 0.84

Safety stock = zσdLT = (0.84)(51.96) = 43.64 or 44 bottles

Reorder point = dL + safety stock = (50)(2) + 44 =144 bottles

Two-Bin System

The two-bin system is a type of continuous review visual system, used to help employees keep track of inventory levels without maintaining formal written records of the inventory’s current position.

How it Works

Divide inventory between bin 1 and bin 2

Bin 1: Use all the inventory in bin 1 first.

Bin 2: Once the inventory in bin 1 runs out, place a new order and then begin using the inventory in bin 2 until the new order arrives.

Summary• Continuous review systems assist management by

informing them when new inventory needs to be ordered and how much inventory to order.

• Reorder points and safety stock can be calculated under 3 circumstances:– Constant demand and lead time

– Variable demand and constant lead time

– Variable demand and lead time

• The two-bin system is a visual Q System

Sources• Chapman, S. N., Clive, L. M., Tony Arnold, J. R. (2008). Introduction to

Materials Management. New Jersey: Pearson Prentice Hall.• Dreckshage, B. J., Kerber B. (2011). Lean Supply Chain Management

Essentials: A Framework for Materials Managers. Florida: Taylor and Francis Group, LLC

• Krajewski, Malhotra, Ritzman (2010). Operantions Management: Processes and Supply Chains. New Jersey: Pearson Prentice Hall.

• Tersine, R. J. (1994). Principles of Inventory Management. New Jersey: PTR Prentice-Hall

• Toomey, J. W. (2000). Inventory Management: Principles, Concepts and Techniques. Massachusetts: Kluwer Academic Publishers