Statistical Process Control

John Oakland, Robert Oakland

• Publisher: Routledge
• Year: 2018
• ISBN:
  9781351662673
• Edition: 7
• Topic: Commerce
• Subtopic: Commerce Général

The business, commercial and public-sector world has changed dramatically since John Oakland wrote the first edition of Statistical Process Control – a practical guide in the mid-eighties. Then people were rediscovering statistical methods of 'quality control' and the book responded to an often desperate need to find out about the techniques and use them on data. Pressure over time from organizations supplying directly to the consumer, typically in the automotive and high technology sectors, forced those in charge of the supplying production and service operations to think more about preventing problems than how to find and fix them. Subsequent editions retained the 'took kit' approach of the first but included some of the 'philosophy' behind the techniques and their use.

The theme which runs throughout the 7th edition is still processes - that require understanding, have variation, must be properly controlled, have a capability, and need improvement - the five sections of this new edition. SPC never has been and never will be simply a 'took kit' and in this book the authors provide, not only the instructional guide for the tools, but communicate the management practices which have become so vital to success in organizations throughout the world. The book is supported by the authors' extensive and latest consulting work within thousands of organisations worldwide.

Fully updated to include real-life case studies, new research based on client work from an array of industries, and integration with the latest computer methods and Minitab software, the book also retains its valued textbook quality through clear learning objectives and end of chapter discussion questions. It can still serve as a textbook for both student and practicing engineers, scientists, technologists, managers and for anyone wishing to understand or implement modern statistical process control techniques.

Part I

Process understanding

1 Quality, processes and control

Objectives

• To introduce the subject of statistical process control (SPC) by considering the basic concepts.

• To define terms such as quality, process and control.

• To distinguish between design quality and conformance quality.

• To define the basics of quality-related costs.

• To set down a system for thinking about SPC and introduce some basic tools.

1.1 The basic concepts

Statistical process control (SPC) is not really about statistics or control, it is about competitiveness. Organizations, whatever their nature, compete on three issues: quality, delivery and price. There cannot be many people in the world who remain to be convinced that the reputation attached to an organization for the quality of its products and services is a key to its success and the future of its employees. Moreover, if the quality is right, the chances are the delivery and price performance will be competitive too.

What is quality?

The word ‘quality’ is often used to signify ‘excellence’ of a product or service – we hear talk about ‘Rolls-Royce quality’ and ‘top quality’. In some manufacturing companies quality may be used to indicate that a product conforms to certain physical characteristics set down with a particularly ‘tight’ specification. But if we are to manage quality it must be defined in a way that recognizes the true requirements of the ‘customer’.

Quality is defined simply as meeting the requirements of the customer and this has been expressed in many ways by other authors:

Fitness for purpose or use (Juran).

The totality of characteristics of an entity that bear upon its ability to satisfy stated and implied needs (ISO 8402, now 9000: 2008).

The total composite product and service characteristics of marketing, engineering, manufacture, and maintenance through which the product and service in use will meet the expectation by the customer (Feigenbaum).

The ability to meet the customer requirements is vital, not only between two separate organizations, but within the same organization. There exists in every factory, every department, every office, a series of suppliers and customers. The PA is a supplier to the boss – is (s)he meeting the requirements consistently? Does the boss receive error-free information set out as (s)he wants it, when (s)he wants it? If so, then we have a quality service. Does the factory receive from its supplier defect-free parts that conform to the requirements of the assembly process every time? If so, then we have a quality supplier.

For industrial and commercial organizations, which are viable only if they provide satisfaction to the consumer, competitiveness in quality is not only central to profitability, but crucial to business survival. The consumer should not be required to make a choice between price and quality, and for manufacturing or service organizations to continue to exist they must learn how to manage quality. In today’s tough and challenging business environment, the development and implementation of a comprehensive quality policy is not merely desirable – it is essential.

Every day people in organizations around the world scrutinize together the results of the examination of the previous day’s production or operations, and commence the ritual battle over whether the output is suitable for the customer. One may be called the Production Manager, the other the Quality Control Manager. They often argue and debate the evidence before them, the rights and wrongs of the specification, and each tries to convince the other of the validity of their argument. Sometimes they nearly break into fighting.

This ritual is associated with trying to answer the question: ‘Have we done the job correctly?’ – ‘correctly’ being a flexible word depending on the interpretation given to the specification on that particular day. This is not quality control, it is post-production/operation detection, wasteful detection of bad output before it hits the customer. There is a belief in some quarters that to achieve quality we must check, test, inspect or measure – the ritual pouring on of quality at the end of the process – and that quality, therefore, is expensive. This is nonsense, but it is frequently still encountered. In the office we find staff checking other people’s work before it goes out, validating computer input data, checking invoices, typing, etc. There is also quite a lot of looking for things, chasing things that are late, apologizing to customers for non-delivery and so on – waste, waste and more waste.

The problems are often a symptom of the real, underlying cause of this type of behaviour, the lack of understanding of quality management. The concentration of inspection effort at the output stage merely shifts the failures and their associated costs from outside the organization to inside. To reduce the total costs of quality, control must be at the point of manufacture or operation; quality cannot be inspected into an item or service after it has been produced. It is essential for cost-effective control to ensure that articles are manufactured, documents are produced, or that services are generated correctly the first time. The aim of process control is the prevention of the manufacture of defective products and the generation of errors and waste in non-manufacturing areas.

To get away from the natural tendency to rush into the detection mode, it is necessary to ask different questions in the first place. We should not ask whether the job has been done correctly, we should ask first: ‘Can we do the job correctly?’ This has wide implications and this book aims to provide some of the tools which should be used to ensure that the answer is ‘Yes’. However, we should realize straight away that such an answer will only be obtained using satisfactory methods, materials, equipment, skills and instruction, and a satisfactory or capable ‘process’.

What is a process?

A process is the transformation of a set of inputs, which can include materials, actions, methods and operations into desired outputs, in the form of products, information, services or – generally – results. In each area or function of an organization there will be many processes taking place. Each process may be analysed by an examination of the inputs and outputs. This will determine the action necessary to improve quality.

The output from a process is that which is transferred to somewhere or to someone – the customer. Clearly, to produce an output that meets the requirements of the customer, it is necessary to define, monitor and control the inputs to the process, which in turn may have been supplied as output from an earlier process. At every supplier–customer interface there resides a transformation process and every single task throughout an organization must be viewed as a process in this way. To begin to monitor and analyse any process, it is necessary first of all to identify what the process is, and what the inputs and outputs are. Many processes are easily understood and relate to known procedures, e.g. drilling a hole, compressing tablets, filling cans with paint, polymerizing a chemical. Others are less easily identified, e.g. servicing a customer, delivering a lecture, storing a product, inputting to a computer. In some situations it can be difficult to define the process. For example, if the process is making a sales call, it is vital to know whether the scope of the process includes obtaining access to the potential customer or client. Defining the scope of a process is vital, since it will determine both the required inputs and the resultant outputs. A simple ‘static’ model of a process is shown in Figure 1.1. This describes the boundaries of the process. ‘Dynamic’ models of processes will be discussed in Chapter 2.

Figure 1.1 A process: SIPOC

Once the process is specified, the suppliers and inputs, outputs and customers (SIPOC) can also be defined, together with the requirements at each of the interfaces (the voice of the customer). Often the most difficult areas in which to do this are in non-manufacturing organization or non-manufacturing parts of manufacturing organizations, but careful use of appropriate questioning methods can release the necessary information. Sometimes this difficulty stems from the previous absence of a precise definition of the requirements and possibilities. Inputs to processes include: equipment, tools, computers or plant required, materials, people (and the inputs they require, such as skills, training, knowledge, etc.); information including the specification for the outputs, methods or procedures instructions and the environment.

Prevention of failure in any transformation is possible only if the process definition, inputs and outputs are properly documented and agreed. This documentation will allow reliable data about the process itself to be collected (the voice of the process), analysis to be performed, and action to be taken to improve the process to prevent failure or non-conformance with the requirements. The target in the operation of any process is the total avoidance of failure. If the objective of no failures or error-free work is not adopted, at least as a target, then certainly it will never be achieved. The key to success is to align the employees of the business, their roles and responsibilities with the organization and its processes. This is the core of process alignment and business process design or re-design (BPR). When an organization focuses on its key processes, that is the value-adding activities and tasks themselves, rather than on abstract issues such as ‘culture’ and ‘participation,’ then the change process can begin in earnest.

Process design and particularly re-design challenges managers to rethink their traditional methods of doing work and commit to a customer-focused process. Many outstanding organizations have achieved and maintained their leadership through process re-design or ‘re-engineering’. Companies using these techniques have reported significant bottom-line results, including better customer relations, reductions in cycle times, time to market, increased productivity, fewer defects/errors and increased profitability. BPR uses recognized techniques for improving business processes and questions the effectiveness of existing structure...