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Quality control

In engineering and manufacturing, quality control or quality engineering is a set of measures taken to ensure that defective products or services are not produced, and that the design meets performance requirements.



Though terms like 'quality engineering' and 'quality assurance' are relatively new, the ideas have existed just as long as the very art of tool manufacture. Simple tools made of rock or bone were subject to familiar modes of failure. They could be fragile, dull where they should be sharp, sharp where they should be dull, etc. When the first specialized craftsmen arose, manufacturing tools for others, the principle of quality control was simple: "let the buyer beware" (caveat emptor).

The first civil engineering projects, however, needed to be built to specifications. For instance, the four sides Great Pyramid of Giza are perpendicular to within 3.5 arcseconds.

Craft and tradespersons

During the Middle Ages, guilds took the responsibility of quality control upon themselves. All practitioners of a particular trade living in a certain area were required to join the corresponding guild, and the guild instituted punishments for members who turned out shoddy products.

Royal governments purchasing materiel were interested in quality control as customers. For instance, King John of England appointed a certain William Wrotham to supervise the construction and repair of ships. Some centuries later, but also in England, Samuel Pepys, Secretary to the Admiralty, appointed multiple such overseers.

Prior to the extensive division of labor and mechanization resulting from the Industrial Revolution, it was possible for a workman to control the quality of his own product. Working conditions then were more conducive to professional pride.

This led to a system in which large groups of men performing a similar type of work were grouped together under the supervision of a foreman who also took on the responsibility to control the quality of work manufactured.

It has taken approximately 80-90 years (in about 20 year intervals) for present day Quality Assurance to be established from inception to attainment of the present day.

Wartime production

During World War I, the manufacturing process became more complex, and the introduction of large numbers of workmen being supervised by a foreman designated to ensure the quality of the work, which was being produced. This period also introduced mass production and piecework , which created quality problems as workmen could now earn more money by the production of extra products, which in turn led to bad workmanship being passed on to the assembly lines.

It was due to the large amount of bad workmanship being produced that the first full time inspectors were introduced into the large-scale modern factory. These full time inspectors were the real beginning of inspection quality control, and this was the beginning the large inspection organizations of the 1920’s and 1930’s, which were separately organised from production and big enough to be headed by superintendents.

The systematic approach to quality started in industrial manufacture during the 1930’s, mostly in the USA, when some attention was given to the cost of scrap and rework . With the impact of mass production, which was required during the Second World War, it became necessary to introduce a more stringent form of QC which can be identified as “Statistical Quality Control”.

This system came about with the realisation that quality cannot be inspected into an item. By extending the inspection phase and making inspection organizations more efficient, it provides inspectors with control tools such as sampling and control charts.

This “Statistical Quality Control” had a significant contribution in that it provided a sampling inspection system rather that a 100 per cent inspection. This type of inspection however did lead to a lack of realisation to the importance of the engineering of product quality.

For example, if you have a basic sampling scheme with an acceptance level of 4%, what happens is you have a ratio of 96% products released onto the market with 4% defective items – this obviously is a fair risk for any company/customer – unless you happen to be one of the four who ends up with one of the 4% defective item s.


In the 1940’s, inspections progressed more to process control, which together became known as quality control. With the introduction of the nuclear and space industries during the 1950’s, attention was focussed on design.

Progressing through into the 60’s and 70’s, the true meaning of quality assurance really became known.

The following is a summary of an item, which was reported in a newspaper in China during 1993:

“Refrigerators are among the most sought after consumer items in China but at a factory in Beijing the products had a reputation for failure.

For years, factory workers complained that many component parts did not meet the required specifications and the end product did not function as required. Complaining workers quoted the plant manager as stating, “Ship it.” Some customers who had waited for up to five years for their appliances were outraged.

One Monday morning as 500 workers looked on, 18 people – including the Plant Manager, Quality Manager, the Engineering Managers and their top staff were taken out to a rice paddy outside the factory and unceremoniously shot to death for committing unpardonable crimes against the people of China.”

Quality assurance

Quality Assurance covers all activities from design, development, production, installation, servicing and documentation, this introduced the rule “Fit for purpose” and “do it right first time”. It includes the regulation of the quality of raw materials, assemblies, products and components; services related to production; and management, production, and inspection processes.

One of the most widely used paradigms for QA management is the PDCA (Plan-Do-Check-Act) approach, also known as the Shewhart Cycle .

Failure testing

A valuable process to perform on a whole consumer product is failure testing , the operation of a product until it fails, often under stresses such as increasing vibration, temperature and humidity. This exposes many unanticipated weaknesses in a product, and the data is used to drive engineering and manufacturing process improvements. Often quite simple changes can dramatically improve product service, such as changing to mould-resistant paint or adding lock-washer placement to the training for new assembly personnel.

Statistical control

Many organizations use statistical process control to bring the organization to Six Sigma levels of quality, in other words, so that the likelihood of an unexpected failure is confined to six standard deviations on the normal distribution. This probability is less than four one-millionths. Items controlled often include clerical task s such as order-entry as well as conventional manufacturing tasks.

Traditional statistical process controls in manufacturing operations usually proceed by randomly sampling and testing a fraction of the output. Variances of critical tolerances are continuously tracked, and manufacturing processes are corrected before bad parts can be produced.

Company quality

During the 1980’s, the concept of “company quality” with the focus on management and people came to the fore. It was realised that, if all departments approached quality with an open mind, success was possible if the management led the quality improvement process.

The company-wide quality approach places an emphasis on three aspects :-

  1. Elements such as controls, job management, adequate processes, performance and integrity criteria and identification of records
  2. Competence such as knowledge, skills, experience, qualifications
  3. Soft elements, such as personnel integrity, confidence, organisational culture , motivation, team spirit and quality relationships.

The quality of the outputs is at risk if any of these three aspects are deficient in any way.

The approach to quality management given here is therefore not limited to the manufacturing theatre only but can be applied to any business activity:

  • Design work
  • Administrative services
  • Consulting
  • Banking
  • Insurance
  • Computer software
  • Retailing
  • Transportation...

It comprises a quality improvement process, which is generic in the sense it can be applied to any of these activities and it establishes a behaviour pattern , which supports the achievement of quality.

This in turn is supported by quality management practices which can include a number of business system s and which are usually specific to the activities of the business unit concerned.

In manufacturing and construction activities, these business practices can be equated to the models for quality assurance defined by the International Standards contained in the ISO 9000 series and the specified Specifications for quality systems.

Still, in the system of Company Quality, the work being carried out was shop floor inspection which did not control the major quality problems. This led to quality assurance or total quality control, which has come into being recently.

Total quality control

Total Quality Control is the most necessary inspection control of all in cases where, despite statistical quality control techniques or quality improvements implemented, sales decrease.

The major problem which leads to a decrease in sales was that the specifications did not include the most important factor, “What the customer required”.

The major characteristics, ignored during the search to improve manufacture and overall business performance were:-

  • Reliability
  • Maintainability
  • Safety

As the most important factor had been ignored, a few refinements had to be introduced:

  1. Marketing had to carry out their work properly and define the customer’s specifications.
  2. Specifications had to be defined to conform to these requirements.
  3. Conformance to specifications i.e. drawings, standards and other relevant documents, were introduced during manufacturing, planning and control.
  4. Management had to confirm all operators are equal to the work imposed on them and holidays, celebrations and disputes did not affect any of the quality levels.
  5. Inspections and tests were carried out, and all components and materials, bought in or otherwise, conformed to the specifications, and the measuring equipment was accurate, this is the responsibility of the QA/QC department.
  6. Any complaints received from the customers were timorously and satisfactorily dealt with.
  7. Feedback from the user/customer is used to review designs.

If the original specification does not reflect the correct quality requirements, quality cannot be inspected or manufactured into the product.

For instance, all parameters for a pressure vessel should include not only the material and dimensions but operating, environmental, safety, reliability and maintainability requirements.

To conclude, the above forms the basis from which the philosophy of Quality Assurance has evolved, and the achievement of quality or the “fitness-for-purpose” is “Quality Awareness” throughout the company.

Related topics

External links

  • Saksoft
  • Open Directory
  • Citations from CiteSeer
  • Software Testing and Quality Control - Outline
  • Linux Kernel Scalable Test Platform
  • OSDL Data Base Test Suite
  • Agility an Issue and Requirements Tracking Software from AgileEdge
  • Open source software testing tools
  • The online community for software testing & quality assurance professionals
  • QACity
  • Stickyminds website for all things connected with software testing - associated with "Better Software" Magazine
  • Methods & Tools: on-line magazine with many articles on software testing and quality


Last updated: 02-07-2005 18:03:42
Last updated: 02-20-2005 20:15:00