31 May 2011

Acceptance Sampling Versus Statistical Process Control

Quality Assurance (SPC) versus Quality Control (Acceptance Sampling) is the subject of the debate that examines which approach should be taken when manufacturing products and striving to attain quality levels that are deemed to be acceptable to both manufacturer and customer.

There are no clear cut answers, there are arguments for and against using one or the other, using both or using neither.

Quality comes at a cost and generally it is accepted that quality assurance methods which include SPC are more complex and therefore more expensive to set up, but once in place tend to have lower running costs and are geared towards ‘zero defects’ rather than an acceptable defect level.

Ref. Edwards & Endean (1990)

Does this mean then that SPC is the answer to which quality process to use. Unfortunately it is not as straightforward as that, one factor that cannot be ignored is that SPC is only effective when a process is deemed to be in control i.e. that the process has been set up correctly and is inherently capable of producing components to meet the design specification.

In order to ensure the process is in this condition it is necessary to take early samples of components produced, measure critical features and use the results to make a decision about the process. It has been argued that this is very similar to Acceptance Sampling and that the data could in fact be used for this purpose as well as to establish if the process is under control. In other words it presents a case for combining the 2 methods.

Edward G Schilling discusses an ABC plan which also supports the view that there is synergy between SPC and acceptance sampling and that they can be combined to present a scenario where there is minimal risk to the consumer.

The ABC plan is subject to several constraints :

• Acceptable quality levels are not utilized
• Acceptance number of zero
• Simplicity

The plan progresses through 3 stages :

• Stage A – control being established
• Stage B – capability being established
• Stage C – capability being maintained

There are various rules associated with the implementation of this plan which are designed to promote continual improvement and learning, but it is accepted that the plan is a prototype and is subject to further development.

Ref. Schilling (1994)

W. Edwards Deming condemns the use of acceptance sampling and proposes all or nothing inspection. Although what he is really objecting to is the misuse of acceptance sampling and the suggestion that a proportion of defective components is acceptable.

In contradiction to this statement it is suggested that Acceptance Sampling is valid for processes that are in a state of chaos and until they reach the point where a process becomes stabilized.

Ref. Sower et al (1993)

There are arguments for both processes either independent of one another or combined. In cases of mass production it is clear that dependent on the situation, who the customers, are what agreements have been reached etc. then each and every option could be assessed to be the most appropriate.

A more modern approach is a move to Total Quality Management which was discussed at the end of the article on Statistical Process Control. What the plan represents is a totally different approach to manufacturing which requires a stepped culture change and a belief in the individual working in a team environment. An approach most modern day management structures would tend to adhere to.

The concept of ‘Poka-Yoke’ otherwise known as ‘mistake-proofing', an approach presented by Shigeo Shingo and refers to the need to prevent defects occurring in the first place by examining the process to determine what can cause problems and introduce mechanisms that will prevent this from happening.

This can be likened to the SPC approach of avoiding defects before they happen but differs in that once proper processes are established there should no longer be a need for ongoing monitoring.

The move to more flexible and adaptable quality systems has become more supportable given that manufacturing has had to become ever more flexible and fluid as the demands of society have moved from acceptance of limited choices of mass produced products to a situation where the individual is looking for variability in the choices they are offered to suit their own personal requirements.

Also the advent of CNC machining and other new manufacturing techniques has provided the means for supporting this level of flexibility. The result of this is smaller production runs and frequent re-tooling to produce the smaller production batches required. Neither SPC or Acceptance Sampling lend themselves to this scenario readily, so alternative methods have had to be identified and developed.

No comments:

Post a Comment