Process Capability vs Process Performance – Understanding the Practical Difference



The most common method for a customer to communicate quality requirements to a supplier is through the use of a capability index. Cp/Cpk and Pp/Ppk are the indices most commonly used. These indices compare the product specification, known as the “Voice of the Customer”, to the process width, known as the “Voice of the Process”. It is commonly said that we are to use Cp and Cpk to represent “short term” variation and Pp and Ppk to represent “long term” variation. Although this definition seems simple enough, there is considerable confusion in the industry regarding how these two sets of indices differ and precisely when to use one versus the other. Attendees will not only learn the mathematical difference between these two sets of metrics, but they will also gain a practical understanding of when to use each and how to interpret the difference.

The presentation will also include a brief overview of variation and basic statistics as well as individual and moving range (I-MR) control charting, both critical prerequisites to gain an advanced understanding of process capability.

Areas to be Covered:

Mathematical understanding of Cp/Cpk & Pp/Ppk
Basic variation and statistics understanding
Individual & Moving Range (I-MR) charts fundamentals
Knowledge of why both Cp/Cpk & Pp/Ppk are needed
Practical application of capability indices
Common pitfalls and misunderstanding

Who will Benefit:

Design Engineers
QA/QC Engineers
Manufacturing Teams
QA/QC Managers
Process Engineers
Research & Development Teams
Everyone involved in specifying or interpreting customer requirements where capability indices are used

Jerry Schmidt

Jerry Schmidt is a licensed professional engineer with over 35 years in the plastics injection molding industry. He has served in numerous capacities including plant manager, director of developmental engineering & quality manager. He was trained and certified as a Six Sigma Black Belt and received his Six Sigma Master Black Belt certification in January 2004.


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