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Quality Control

The ISO 9000 definition states that quality control is part of quality management focused on fulfilling quality requirements (requirements related to quality).

What the definition fails to tell us is that controls regulate performance. They prevent change and when applied to quality regulate quality performance and prevent undesirable changes in the quality standards. Quality control is a process for maintaining standards and not for creating them. Standards are maintained through a process of selection, measurement and correction of work, so that only those products or services which emerge from the process meet the standards. In simple terms quality control prevents undesirable changes being present in the quality of the product or service being supplied. The simplest form of quality control is illustrated in the Figure below. Quality control can be applied to particular products, to processes which produce the products or to the output of the whole organization by measuring the overall quality performance of the organization

Quality control is often regarded as a post event activity. i.e. a means of detecting whether quality has been achieved and taking action to correct any deficiencies. However, one can control results by installing sensors before, during or after the results are created. It all depends on where you install the sensor, what you measure and the consequences of failure. Some failures cannot be allowed to occur and so must be prevented from happening through rigorous planning and design. Other failures are not so critical but must be corrected immediately using automatic controls or fool proofing. Where the consequences are less severe or where other types of sensor are not practical or possible, human inspection and test can be used as a means of detecting failure. Where failure cannot be measured without observing trends over longer periods, one can use information controls. They do not stop immediate operations but may well be used to stop further operations when limits are exceeded. If you have no controls then quality products are produced by chance and not design. The more controls you install the more certain you are of producing products of consistent quality but there is balance to be achieved. Beware of the law of diminishing returns. 

It is often deemed that quality assurance serves prevention and quality control detection, but a control installed to detect failure before it occurs serves prevention such as reducing the tolerance band to well within the specification limits. So quality control can prevent failure. Assurance is the result of an examination whereas control produces the result. Quality Assurance does not change the product, Quality Control does.

Quality Control is also a term used as a name of a department. In most cases Quality Control Departments perform inspection and test activities and the name derives from the authority that such departments have been given. They sort good products from bad products and authorize the release of the good products. It is also common to find that Quality Control Departments perform supplier control activities which are called Supplier Quality Assurance or Vendor Control. In this respect they are authorized to release products from suppliers into the organization either from the supplier's premises or on receipt in the organization.

Since to control anything requires the ability to effect change, the title Quality Control Department is in fact a misuse of the term since such departments do not in fact control quality. They do act as a regulator if given the authority to stop release of product, but this is control of supply and not of quality. Authority to change product usually remains in the hands of the producing departments. It is interesting to note that similar activities within a Design Department are not called quality control but Design Assurance or some similar term. Quality Control has for decades been a term applied primarily in the manufacturing areas of an organization and hence it is difficult to change peoples perceptions after so many years of the terms incorrect use.

In recent times the inspection and test activities have been transferred into the production departments of organizations, sometimes retaining the labels and sometimes reverting to the inspection and test labels.

Control of quality, or anything else for that matter, can be accomplished by the following steps:

  1. Determine what parameter is to be controlled.
  2. Establish its criticality and whether you need to control before, during or after results are produced (this can be done by undertaking a risk assessment)
  3. Establish a specification for the parameter to be controlled which provides limits of acceptability and units of measure.
  4. Produce plans for control which specify the means by which the characteristics will be achieved and variation detected and removed.
  5. Organize resources to implement the plans for quality control.
  6. Install a sensor at an appropriate point in the process to sense variance from specification.
  7. Collect and transmit data to a place for analysis.
  8. Verify the results and diagnose the cause of variance.
  9. Propose remedies and decide on the action needed to restore the status quo. Take the agreed action and check that the variance has been corrected.

More information about quality control can be found in Chapters 9 & Parts 8 & 9 of the ISO 9000 Quality Systems Handbook 7E

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