Capability Analysis

Q: Why is a standard capability analysis determined to be best represented by 30 pieces?

I have answered this question by explaining it best represents a normal distribution. But I wonder if this is traceable to an industry standard?

A: You are right that most people associate 30 pieces with the conventional quantity for performing a capability study.  Although I don’t know the origin of this number, I can tell you the following:

  • The number 30 has nothing to do with whether or not the population is normally distributed.
  • In many applications, the number 30 is insufficient to properly model the process.  For example, automotive industry standards published by the Automotive Industry Action Group (AIAG) in their statistical process control (SPC) and production part approval process (PPAP) documents define 100 pieces as the appropriate sample size for an initial capability study (based on 20 subgroups of five or 25 subgroups of four).

I hope you find this helpful.

Denis J. Devos, P.Eng
A Fellow of the American Society for Quality
Devos Associates Inc.
London Ontario

Related Content:

Statistics in Pharmaceutical Development and ManufacturingJournal of Quality Technology,  open access

An overview is given of the use of statistical thinking and methods in the research and development and manufacturing functions in the pharmaceutical industry. Four case studies illustrate how these issues work in real life settings. A synopsis of these issues concludes that the technical nature of pharmaceutical development and manufacturing offers opportunities for the effective use of statistical methods leading to both process-development understanding and product-quality improvement.

Build a Usable Process Capability Database, Six Sigma Forum Magazine, Open Access

Design for Six Sigma requires that designs meet customer needs without sacrificing quality. A number of statistical tools can be used to produce process capability data to enable development teams to design products that can be produced at reasonable cost on existing equipment. However, setting up and using a process capability database is poorly understood and as a result, it is seldom used. To create a successful database, it is necessary to get management support, build the right data structure, collect the right data, and use the data correctly. A correctly designed database will allow the product development team to focus its efforts only on those tolerances in which the capability is unclear or in which function or cost improvements can be achieved.

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