通常，我们谈及 Quality（质量）时，最常见的问题就是：Quality 是什么？
- 符合要求（Conformance To Requirements）：质量就是指产品或服务的可测量的属性满足一组固定的要求，也就是说，符合预先定义的要求。
- 满足客户需求（Meeting Customer Needs）：质量独立于任何可测量的属性，也就是说，质量就是指产品或服务满足客户预期的能力，无论是显式的还是隐式的。
- Philip B. Crosby 的质量理论
- Walter Edwards Deming 的质量理论
- Armand Vallin Feigenbaum 的质量理论
- Kaoru Ishikawa 的质量理论
- Joseph Moses Juran 的质量理论
- Walter Andrew Shewhart 的质量理论
在 Philip B. Crosby 的 “Quality is free: the art of making quality certain” 书中写到：
The first erroneous assumption is that quality means goodness, or luxury or shininess. The word “quality” is often used to signify the relative worth of something in such phrases as “good quality”, “bad quality” and “quality of life” - which means different things to each and every person. As follows quality must be defined as “conformance to requirements” if we are to manage it. Consequently, the nonconformance detected is the absence of quality, quality problems become nonconformance problems, and quality becomes definable.
Crosby 很明确的将质量定义为 “Conformance to requirements” 。尽管如此，他也指出要尝试尽可能的理解客户的全部预期。同时，Crosby 也强调了清晰定义质量的可可测量和可管理的概念的重要性。
Crosby 关于质量管理的 4 项基本原则：
- Quality is conformance to requirements
- Quality prevention is preferable to quality inspection
- Zero defects is the quality performance standard
- Quality is measured in monetary terms – the price of non-conformance
Crosby 关于质量改进的 14 个步骤：
- Management is committed to quality – and this is clear to all
- Create quality improvement teams – with (senior) representatives from all departments.
- Measure processes to determine current and potential quality issues.
- Calculate the cost of (poor) quality
- Raise quality awareness of all employees
- Take action to correct quality issues
- Monitor progress of quality improvement – establish a zero defects committee.
- Train supervisors in quality improvement
- Hold “zero defects” days
- Encourage employees to create their own quality improvement goals
- Encourage employee communication with management about obstacles to quality
- Recognise participants’ effort
- Create quality councils
- Do it all over again – quality improvement does not end
Walter Edwards Deming 在 “Out of the crisis: quality, productivity and competitive position” 一书中指出：
The problem inherent in attempts to define the quality of a product, almost any product, where stated by the master Walter A. Shewhart. The difficulty in defining quality is to translate future needs of the user into measurable characteristics, so that a product can be designed and turned out to give satisfaction at a price that the user will pay. This is not easy, and as soon as one feels fairly successful in the endeavor, he finds that the needs of the consumer have changed, competitors have moved in etc.
Deming 强调质量必须以满足客户为基准（也就是：“meeting customer needs”），概念上要比 “conformance to requirements” 更广泛些。
Deming 企业管理职能转变的 14 条原则：
- Create constancy of purpose for improvement of product and service.
- Adopt the new philosophy.
- Cease dependence on mass inspection.
- End awarding business on price.
- Institute training.
- Institute leadership.
- Drive out fear.
- Break down barriers between departments.
- Eliminate slogans, exhortations and numerical targets.
- Eliminate numerical quotas or work standards.
- Remove barriers to taking pride in workmanship.
- Institute a vigorous programme of education.
- Take action to accomplish the transformation.
随着全面质量控制的深渊影响，现在 “Feigenbaum” 和 “Total Quality Control” 几乎变成了同义词。
Armand Vallin Feigenbaum 是这样描述他对质量控制的观点的：
Quality is a customer determination, not an engineer’s determination, not a marketing determination, nor a general management determination. It is based on upon the customer’s actual experience with the product or service, measured against his or her requirements – stated or unstated, conscious or merely sensed, technically operational or entirely subjective – and always representing a moving target in a competitive market. Product and service quality can be defined as: The total composite product and service characteristics of marketing, engineering, manufacture and maintenance though witch the product and service in use will meet the expectations of the customer.
很显然，Feigenbaum 关于质量的定义毋庸置疑也是 “meeting customer needs”。而事实上，他也充分强调同时满足客户的实际需求和预期需求的重要性，也就是说满足要同时满足客户的显式需求和隐式需求。
Kaoru Ishikawa 在他的书 “What is quality control? The Japanese Way” 中写道：
We engage in quality control in order to manufacture products with the quality which can satisfy the requirements of consumers. The mere fact of meeting national standards or specifications is not the answer, it is simply insufficient. International standards established by the International Organization for Standardization (ISO) or the International Electrotechnical Commission (IEC) are not perfect. They contain many shortcomings. Consumers may not be satisfied with a product which meets these standards. We must also keep in mind that consumer requirements change from year to year and even frequently updated standards cannot keep the pace with consumer requirements. How one interprets the term “quality” is important. Narrowly interpreted, quality means quality of products. Broadly interpreted, quality means quality of product, service, information, processes, people, systems etc. etc.
Ishikawa 关于质量的观点也符合 “meeting customer needs” 的定义。他强调质量是一个动态的概念，质量会随着客户需求和预期的改变保持持续的变化。
在 “Jurans’s Quality Control Handbook” 一书中，Joseph M. Juran 指出了两点质量的涵义：
The word quality has multiple meanings. Two of those meanings dominate the use of the word:
1) Quality consists of those product features which meet the need of customers and thereby provide product satisfaction.
2) Quality consists of freedom from deficiencies. Nevertheless, in a handbook such as this it is most convenient to standardize on a short definition of the word quality as “fitness for use”.
Juran 关于质量的定义，与前面几位质量大师有几分区别。他的观点是，我们不能使用 “Quality” 这个词来包含满足客户需求或预期，因为它太难实现了。取而代之的是，使用 “fitness for use” 来描述需求和产品属性。看起来，Juran 的质量定义可以解释成 “conformance to requirements” 而不是 “meeting customer needs”。
- Quality Planning
- Quality Control
- Quality Improvement
Walter A. Shewhart 在 “Economic control of quality of manufactured product” 一书中定义了质量：
There are two common aspects of quality: One of them has to do with the consideration of the quality of a thing as an objective reality independent of the existence of man. The other has to do with what we think, feel or sense as a result of the objective reality. In other word, there is a subjective side of quality.
尽管 Shewhart 关于质量的定义始于19世纪20年代，但其仍被认为是非常优秀的定义。Shewhart 谈到了质量的主观性和客观性，所以很合理的同时满足了 “conformance to requirements” 和 “meeting customer needs”。
- Philip B. Crosby
- W. Edwards Deming
- Armand V. Feigenbaum
- Kaoru Ishikawa
- Joseph M. Juran
- Walter A. Shewhart