Nigel S., Chambers S., Johnson R. Operations Management
Подождите немного. Документ загружается.
Although annual usage and value are the two criteria most commonly used to determine
a stock classification system, other criteria might also contribute towards the (higher) classifica-
tion of an item:
● Consequence of stock-out. High priority might be given to those items which would seriously
delay or disrupt other operations, or the customers, if they were not in stock.
● Uncertainty of supply. Some items, although of low value, might warrant more attention
if their supply is erratic or uncertain.
● High obsolescence or deterioration risk. Items which could lose their value through
obsolescence or deterioration might need extra attention and monitoring.
Some more complex stock classification systems might include these criteria by classifying
on an A, B, C basis for each. For example, a part might be classed as A/B/A meaning it is an
A category item by value, a class B item by consequence of stock-out and a class A item by
obsolescence risk.
Part Three Planning and control
364
of the total inventory value. As a part, however, it is only one-twentieth or 5 per cent of
the total number of items stocked. This item together with the next highest value item
(D/012) accounts for only 10 per cent of the total number of items stocked, yet accounts
for 47.37 per cent of the value of the stock, and so on.
This is shown graphically in Figure 12.16. Here the wholesaler has classified the
first four part numbers (20 per cent of the range) as Class A items and will monitor the
usage and ordering of these items very closely and frequently. A few improvements in
order quantities or safety stocks for these items could bring significant savings. The six
next, part numbers C/375 through to A/138 (30 per cent of the range), are to be treated
as Class B items with slightly less effort devoted to their control. All other items are
classed as Class C items whose stocking policy is reviewed only occasionally.
Figure 12.16 Pareto curve for items in a warehouse
M12_SLAC0460_06_SE_C12.QXD 10/20/09 9:45 Page 364
Measuring inventory
In our example of ABC classifications we used the monetary value of the annual usage of
each item as a measure of inventory usage. Monetary value can also be used to measure the
absolute level of inventory at any point in time. This would involve taking the number
of each item in stock, multiplying it by its value (usually the cost of purchasing the item)
and summing the value of all the individual items stored. This is a useful measure of the
investment that an operation has in its inventories but gives no indication of how large that
investment is relative to the total throughput of the operation. To do this we must compare
the total number of items in stock against their rate of usage. There are two ways of doing
this. The first is to calculate the amount of time the inventory would last, subject to normal
demand, if it were not replenished. This is sometimes called the number of weeks’ (or days’,
months’, years’, etc.) cover of the stock. The second method is to calculate how often the
stock is used up in a period, usually one year. This is called the stock turn or turnover of stock
and is the reciprocal of the stock-cover figure mentioned earlier.
Stock turn
Chapter 12 Inventory planning and control
365
This approach to inventory classification can sometimes be misleading. Many professional
inventory managers point out that the Pareto law is often misquoted. It does not say
that 80 per cent of the SKUs (stock-keeping units) account for only 20 per cent inventory
value. It accounts for 80 per cent of inventory ‘usage’ or throughput value, in other words
sales value. In fact it is the slow-moving items (the C category items) that often pose the
greatest challenge in inventory management. Often these slow-moving items, although
only accounting for 20 per cent of sales, require a large part (typically between one-half
and two-thirds) of the total investment in stock. This is why slow-moving items are a real
problem. Moreover, if errors in forecasting or ordering result in excess stock in ‘A class’
fast-moving items, it is relatively unimportant in the sense that excess stock can be sold
quickly. However, excess stock in a slow-moving C item will be there a long time. According
to some inventory managers, it is the A items that can be left to look after themselves, it
is the B and even more the C items that need controlling.
Critical commentary
A small specialist wine importer holds stocks of three types of wine, Chateau A,
Chateau B and Chateau C. Current stock levels are 500 cases of Chateau A, 300 cases
of Chateau B and 200 cases of Chateau C. Table 12.6 shows the number of each held in
stock, their cost per item and the demand per year for each.
The total value of stock =Σ(average stock level × cost per item)
= (500 × 3) + (300 × 4) + (200 × 5)
= 3,700
The amount of stock cover provided by each item stocked is as follows (assuming 50 sales
weeks per year):
Chateau A, stock cover ==×50 = 12.5 weeks
Chateau B, stock cover ==×50 = 10 weeks
Chateau C, stock cover ==×50 = 10 weeks
200
1,000
stock
demand
300
1,500
stock
demand
500
2,000
stock
demand
Worked example
➔
M12_SLAC0460_06_SE_C12.QXD 10/20/09 9:45 Page 365
Inventory information systems
Most inventories of any significant size are managed by computerized systems. The many
relatively routine calculations involved in stock control lend themselves to computerized
support. This is especially so since data capture has been made more convenient through
the use of bar-code readers and the point-of-sale recording of sales transactions. Many com-
mercial systems of stock control are available, although they tend to share certain common
functions.
Updating stock records
Every time a transaction takes place (such as the sale of an item, the movement of an
item from a warehouse into a truck, or the delivery of an item into a warehouse) the posi-
tion, status and possibly value of the stock will have changed. This information must be
recorded so that operations managers can determine their current inventory status at
any time.
Generating orders
The two major decisions we have described previously, namely how much to order and
when to order, can both be made by a computerized stock control system. The first decision,
setting the value of how much to order (Q), is likely to be taken only at relatively infrequent
intervals. Originally almost all computer systems automatically calculated order quantities
Part Three Planning and control
366
The stock turn for each item is calculated as follows:
Chateau A, stock turn ===4 times/year
Chateau B, stock turn ===5 times/year
Chateau C, stock turn ===5 times/year
To find the average stock cover or stock turn for the total items in the inventory, the
individual item measures can be weighted by their demand levels as a proportion of total
demand (4,500). Thus:
Average stock cover = 12.5 ×+10 × ) + 10 ×
= 11.11
Average stock turn = 4 ×+5 ×+5 ×
= 4.56
D
F
1,000
4,500
A
C
D
F
1,500
4,500
A
C
D
F
2,000
4,500
A
C
D
F
1,000
4,500
A
C
1,500
4,500
A
C
D
F
2,000
4,500
A
C
1,000
200
demand
stock
1,500
300
demand
stock
2,000
500
demand
stock
Table 12.6 Stock, cost and demand for three stocked items
Item Average number in stock Cost per item (£) Annual demand
Chateau A 500 3.00 2,000
Chateau B 300 4.00 1,500
Chateau C 200 5.00 1,000
M12_SLAC0460_06_SE_C12.QXD 10/20/09 9:45 Page 366
by using the EOQ formulae covered earlier. Now more sophisticated algorithms are used,
often using probabilistic data and based on examining the marginal return on investing in
stock. The system will hold all the information which goes into the ordering algorithm but
might periodically check to see if demand or order lead times, or any of the other parameters,
have changed significantly and recalculate Q accordingly. The decision on when to order, on
the other hand, is a far more routine affair which computer systems make according to what-
ever decision rules operations managers have chosen to adopt: either continuous review or
periodic review. Furthermore, the systems can automatically generate whatever documentation
is required, or even transmit the re-ordering information electronically through an electronic
data interchange (EDI) system.
Generating inventory reports
Inventory control systems can generate regular reports of stock value for the different
items stored, which can help management monitor its inventory control performance.
Similarly, customer service performance, such as the number of stock-outs or the number
of incomplete orders, can be regularly monitored. Some reports may be generated on an
exception basis. That is, the report is only generated if some performance measure deviates
from acceptable limits.
Forecasting
Inventory replenishment decisions should ideally be made with a clear understanding of
forecast future demand. The inventory control system can compare actual demand against
forecast and adjust the forecast in the light of actual levels of demand. Control systems of this
type are treated in more detail in Chapter 14.
Common problems with inventory systems
Our description of inventory systems has been based on the assumption that operations
(a) have a reasonably accurate idea of costs such as holding cost, or order cost, and (b) have
accurate information that really does indicate the actual level of stock and sales. But data
inaccuracy often poses one of the most significant problems for inventory managers. This is
because most computer-based inventory management systems are based on what is called the
perpetual inventory principle. This is the simple idea that stock records are (or should be)
automatically updated every time that items are recorded as having been received into an
inventory or taken out of the inventory. So,
opening stock level
++
receipts in
−−
dispatches out
==
new stock level.
Any errors in recording these transactions and/or in handling the physical inventory
can lead to discrepancies between the recorded and actual inventory, and these errors are
perpetuated until physical stock checks are made (usually quite infrequently). In practice
there are many opportunities for errors to occur, if only because inventory transactions are
numerous. This means that it is surprisingly common for the majority of inventory records
to be in inaccurate. The underlying causes of errors include:
● keying errors: entering the wrong product code
● quantity errors: a mis-count of items put into or taken from stock
● damaged or deteriorated inventory not recorded as such, or not correctly deleted from the
records when it is destroyed
● the wrong items being taken out of stock, but the records not being corrected when they
are returned to stock
● delays between the transactions being made and the records being updated
● items stolen from inventory (common in retail environments, but also not unusual in
industrial and commercial inventories).
Perpetual inventory
principle
Chapter 12 Inventory planning and control
367
M12_SLAC0460_06_SE_C12.QXD 10/20/09 9:45 Page 367
Part Three Planning and control
368
Summary answers to key questions
Check and improve your understanding of this chapter using self assessment questions
and a personalised study plan, audio and video downloads, and an eBook – all at
www.myomlab.com.
➤ What is inventory?
■ Inventory, or stock, is the stored accumulation of the transformed resources in an operation.
Sometimes the words ‘stock’ and ‘inventory’ are also used to describe transforming resources,
but the terms stock control and inventory control are nearly always used in connection with
transformed resources.
■ Almost all operations keep some kind of inventory, most usually of materials but also of
information and customers (customer inventories are normally called ‘queues’).
➤ Why is inventory necessary?
■ Inventory occurs in operations because the timing of supply and the timing of demand do not
always match. Inventories are needed, therefore, to smooth the differences between supply
and demand.
■ There are five main reasons for keeping inventory:
– to cope with random or unexpected interruptions in supply or demand (buffer inventory);
– to cope with an operation’s inability to make all products simultaneously (cycle inventory);
– to allow different stages of processing to operate at different speeds and with different
schedules (de-coupling inventory);
– to cope with planned fluctuations in supply or demand (anticipation inventory);
– to cope with transportation delays in the supply network (pipeline inventory).
➤ What are the disadvantages of holding inventory?
■ Inventory is often a major part of working capital, tying up money which could be used more
productively elsewhere.
■ If inventory is not used quickly, there is an increasing risk of damage, loss, deterioration, or
obsolescence.
■ Inventory invariably takes up space (for example, in a warehouse), and has to be managed,
stored in appropriate conditions, insured and physically handled when transactions occur. It
therefore contributes to overhead costs.
➤ How much inventory should an operation hold?
■ This depends on balancing the costs associated with holding stocks against the costs associ-
ated with placing an order. The main stock-holding costs are usually related to working capital,
whereas the main order costs are usually associated with the transactions necessary to gener-
ate the information to place an order.
■ The best-known approach to determining the amount of inventory to order is the economic
order quantity (EOQ) formula. The EOQ formula can be adapted to different types of inventory
profile using different stock behaviour assumptions.
■ The EOQ approach, however, has been subject to a number of criticisms regarding the true cost
of holding stock, the real cost of placing an order, and the use of EOQ models as prescriptive
devices.
M12_SLAC0460_06_SE_C12.QXD 10/20/09 9:45 Page 368
Chapter 12 Inventory planning and control
369
➤ When should an operation replenish its inventory?
■ Partly this depends on the uncertainty of demand. Orders are usually timed to leave a certain
level of average safety stock when the order arrives. The level of safety stock is influenced by
the variability of both demand and the lead time of supply. These two variables are usually
combined into a lead-time usage distribution.
■ Using re-order level as a trigger for placing replenishment orders necessitates the continual
review of inventory levels. This can be time-consuming and expensive. An alternative approach
is to make replenishment orders of varying size but at fixed time periods.
➤ How can inventory be controlled?
■ The key issue here is how managers discriminate between the levels of control they apply to
different stock items. The most common way of doing this is by what is known as the ABC
classification of stock. This uses the Pareto principle to distinguish between the different values
of, or significance placed on, types of stock.
■ Inventory is usually managed through sophisticated computer-based information systems
which have a number of functions: the updating of stock records, the generation of orders, the
generation of inventory status reports and demand forecasts. These systems critically depend
on maintaining accurate inventory records.
Trans-European Plastics (TEP) is one of Europe’s largest
manufacturers of plastic household items. Its French
factory makes a range of over 500 products that are sold
to wholesalers and large retailers throughout Europe. The
company dispatches orders within 24 hours of receipt
using an international carrier. All customers would expect
to receive their requirements in full within one week. The
manufacturing operation is based on batch production,
employing 24 large injection-moulding machines. Weekly
production schedules are prepared by the Planning and
Control office, detailing the sequence of products (moulds
and colours) to be used, the quantity required for each
batch, and the anticipated timing of each production run.
Mould changes (‘set-ups’) take on average three hours, at
an estimated cost of A500 per set-up.
Concerned about the declining delivery reliability,
increased levels of finished goods inventory and falling pro-
ductivity (apparently resulting from ‘split batches’ where only
part of a planned production batch is produced to over-
come immediate shortages), the CEO, Francis Lamouche,
employed consultants to undertake a complete review of
operations. On 2 January, a full physical inventory check
was taken. A representative sample of 20 products from
the range is shown in Table 12.7.
Because of current high demand for many products,
the backlog of work for planned stock replenishment
Case study
Trans-European Plastics
currently averages two weeks, and so all factory orders
must be planned at least that far in advance. The re-order
quantities (see Table 12.7) had always been established
by the Estimating Department at the time when each
new product was designed and the manufacturing costs
were established, based on Marketing’s estimates of likely
demand. Recently, however, to minimize the total cost of
set-ups and to maximize capacity utilization, all products
are planned for a minimum production run of 20 hours.
The individual re-order levels have not been reviewed
for several years, but were originally based on two weeks’
average sales at that time. About 20 per cent of the
➔
Source: Alamy/ArchivBerlin Fotoagentur GmbH
M12_SLAC0460_06_SE_C12.QXD 10/20/09 9:45 Page 369
products are very seasonal (e.g. Garden Range), with peak
demand from April to August. Storage bins sell particularly
well from October to December.
The European Marketing Manager summarized the
current position, ‘Our coverage of the market has never
been so comprehensive; we are able to offer a full range of
household plastics, which appeals to most European tastes.
But we will not retain our newly developed markets unless
we can give distributors confidence that we will supply all
their orders within one week. Unfortunately, at the moment,
many receive several deliveries for each order, spread over
many weeks. This certainly increases their administrative
and handling costs, and our haulage costs. And sometimes
the shortfall is only some small, low-value items like clothes
pegs.’
The factory operates on three seven-hour shifts, Monday
to Friday: 105 hours per week, for 50 weeks per year. Regular
overtime, typically 15 hours on a Saturday, has been worked
most of the last year. Sunday is never used for production,
allowing access to machines for routine and major overhauls.
Machines are laid out in groups so that each operator can
be kept highly utilized, attending to at least four machines.
Any product can be made on any machine.
Pierre Dumas, the production manager, was concerned
about storage space: ‘At the moment our warehouse is
full, with products stacked on the floor in every available
corner, which makes it vulnerable to damage from passing
forklifts and from double-handling. We have finally agreed
to approve an extension (costing over one million Euros)
to be constructed in June–September this year, which will
replace contract warehousing and associated transport
which is costing us about 5 per cent of the manufacturing
costs of the stored items. The return on investment for this
project is well above our current 8 per cent cost of capital.
There is no viable alternative, because if we run out of
space, production will have to stop for a time. Some of our
products occupy very large volumes of rack space. How-
ever, in the meantime we have decided to review all the
re-order quantities. They seem either to result in excessive
stock or too little stock to provide the service required.
Large items such as the Baby Bath (Item 143BB) could
be looked at first. This is a good starting point because
the product has stable and non-seasonal demand. We
estimate that it costs us around 20 per cent of the manu-
facturing variable costs to store such items for one year.’
Questions
1 Why is TEP unable to deliver all its products reliably
within the target of one week, and what effects might
that have on the distributors?
2 Applying the EBQ model, what batch size would you
recommend for this product? How long will each batch
take to produce, and how many batches per year will be
made? Should this model be applied to calculate the
re-order quantity for all the products, and if not, why?
3 How would the EBQ change if the set-up costs
were reduced by 50 per cent, and the holding costs
were reassessed at 40 per cent, taking account of the
opportunity costs of capital at TEP?
Part Three Planning and control
370
Table 12.7 Details of a representative sample of 20 TEP products
Product Description Unit manuf’g Last 12 mths’ Physical Re-order Standard
reference variable cost sales inventory 2 Jan quantity moulding rate**
number* (Euro) (000s) (000s) (000s) (items/hour))
016GH Storage bin large 2.40 10 0 5 240
033KN Storage jar + lid 3.60 60 6 4 200
041GH 10 litre bucket 0.75 2,200 360 600 300
062GD Grecian-style pot 4.50 40 15 20 180
080BR Bathroom mirror 7.50 5 6 5 250
101KN 1 litre jug 0.90 100 22 20 600
126KN Pack (10) bag clips 0.45 200 80 50 2,000
143BB Baby bath 3.75 50 1 2 90
169BB Baby potty 2.25 60 0 4 180
188BQ Barbecue table 16.20 10 8 5 120
232GD Garden bird bath 3.00 2 6 4 200
261GH Broom head 1.20 60 22 20 400
288KN Pack (10) clothes pegs 1.50 10 17 50 1,000
302BQ Barbecue salad fork 0.30 5 12 8 400
351GH Storage bin small 1.50 25 1 6 300
382KN Round mixing bowl 0.75 800 25 80 650
421KN Pasta jar 3.00 1 3 5 220
444GH Wall hook 0.75 200 86 60 3,000
472GH Dustbin + lid 9.00 300 3 10 180
506BR Soap holder 1.20 10 9 20 400
*The reference number uses the following codes for ranges:
BB = Babycare BQ = Barbecue BR = Bathroom GD = Garden GH = General household KN = Kitchen
**Moulding rate is for the product as described (e.g. includes lids, or pack quantities).
M12_SLAC0460_06_SE_C12.QXD 10/20/09 9:45 Page 370
4 What internal problems result from the current planning
and control policies? In particular, analyse stock turns
and availability (e.g. high and low levels).
5 Using Pareto analysis, categorize the products into
Classes A,B,C, based on usage value. Would this
approach be useful for categorizing and controlling
stock levels of all the products at TEP?
6 What overall recommendations would you make to
Francis Lamouche about the proposed investment in
the warehouse extension?
Chapter 12 Inventory planning and control
371
These problems and applications will help to improve your analysis of operations. You
can find more practice problems as well as worked examples and guided solutions on
MyOMLab at
www.myomlab.com.
An electronics circuit supplier buys microchips from a large manufacturer. Last year the company supplied
2,000 specialist D/35 chips to customers. The cost of placing an order is $50 and the annual holding cost is
estimated to be $2.4 per chip per year. How much should the company order at a time, and what is the total
cost of carrying inventory of this product?
Supermedicosupplies.com is an Internet supplier of medical equipment. One of its most profitable lines is
the ‘Thunderer’ stethoscope. Demand for this product is 15,000 per year, the cost of holding the product is
estimated to be A25 per year and the cost of placing an order A75. How many stethoscopes should the
company order at a time?
Supermedicosupplies.com works a 44-week year. If the lead time between placing an order for stethoscopes
and receiving them is two weeks, what is the re-order point for the Thunderer stethoscopes?
The Super Pea Canning Company produces canned peas. It uses 10,000 litres of green dye per month.
Because of the hazardous nature of this product it needs special transport; therefore the cost of placing an
order is A2,000. If the storage costs of holding the dye are A5 per litre per month, how much dye should be
ordered at a time?
In the example above, if the storage costs of keeping the dye reduce to A3 per litre per month, how much will
inventory costs reduce?
Obtain the last few years’ Annual Report and Accounts (you can usually download these from the company’s
web site) for two materials-processing operations (as opposed to customer or information processing
operations) within one industrial sector. Calculate each operation’s stock–turnover ratio and the proportion
of inventory to current assets over the last few years. Try to explain what you think are the reasons for any
differences and trends you can identify and discuss the likely advantages and disadvantages for the
organizations concerned.
6
5
4
3
2
1
Problems and applications
DeHoratius, N. and Raman, Ananth (2008) Inventory Record
Inaccuracy: An Empirical Analysis, University of Chicago,
http://faculty.chicagobooth.edu/nicole.dehoratius/research.
Viale, J.D. (1997) The Basics of Inventory Management, Crisp
Publications, Menlo Park, Calif. Very much ‘the basics’, but
that is exactly what most people need.
Waters, D. (2003) Inventory Control and Management, John
Wiley and Sons Ltd, Chichester. Conventional but useful
coverage of the topic.
Wild, T. (2002) Best Practice in Inventory Management,
Butterworth-Heinemann. A straightforward and readable
practice-based approach to the subject.
Selected further reading
M12_SLAC0460_06_SE_C12.QXD 10/20/09 9:45 Page 371
Part Three Planning and control
372
www.inventoryops.com/dictionary.htm A great source for
information on inventory management and warehouse
operations.
www.mapnp.org/libary/ops mgnt/ops mgnt.htm General
‘private’ site on operations management, but with some
good content.
www.apics.org Site of APICS: a US ‘educational society for
resource managers’.
www.inventorymanagement.com Site of the Centre for
Inventory Management. Cases and links.
www.opsman.org Lots of useful stuff.
Useful web sites
Now that you have finished reading this chapter, why not visit MyOMLab at
www.myomlab.com where you’ll find more learning resources to help you
make the most of your studies and get a better grade?
M12_SLAC0460_06_SE_C12.QXD 10/20/09 9:45 Page 372
Introduction
Operations managers have to look beyond an internal view if
they want to manage their operations effectively. As operations
outsource many of their activities and buy more of their services
and materials from outside specialists, the way they manage the
supply of products and services to their operations becomes
increasingly important, as does the integration of their distribution
activities. Even beyond this immediate supply chain, there are
benefits from managing the flow between customers’ customers
and suppliers’ suppliers. This activity is now commonly termed
supply chain management. In Chapter 6 we raised the strategic
and structural issues of supply network management; this
chapter considers the more ‘infrastructural’ issues of planning
and controlling the individual chains in the supply network.
Chapter 13
Supply chain planning
and control
Key questions
➤ What is supply chain management?
➤ What are the activities of supply
chain management?
➤ What are the types of relationship
between operations in supply
chains?
➤ How do supply chains behave in
practice?
➤ How can supply chains be
improved?
Figure 13.1 This chapter covers supply chain planning and control
Check and improve your understanding of this chapter using self assessment
questions and a personalised study plan, audio and video downloads, and an
eBook – all at www.myomlab.com.
M13_SLAC0460_06_SE_C13.QXD 10/20/09 9:45 Page 373