Measurement issues can be prevalent at all levels of an organization. To add to this dilemma, the basic calculation and presentation of metrics can sometimes be deceiving. Organizations often state that suppliers must meet process capability objectives, typically measured in Cp, Cpk, Pp and Ppk. The requesters of these objectives often do not realize, however, that these reported numbers can be highly dependent upon how data is collected and interpreted. Also, these process capability metrics typically are utilized only at a component part level. To resolve these issues, practitioners need a common, easy-to-use fundamental measurement for making process stability and capability assessments at all levels of a business, independent of who is making the assessment – something beyond Cp, Cpk, Pp and Ppk.

Cpk is one of many capability metrics that are available. When capability metrics are used, organizations typically provide a Cp and a Cpk. In this paper we will discuss the mechanics of these two capability metrics, along with the pros and cons. In summary, the Cpk can provide insight on performance to a requirement if the process data used in the calculation comes from a normal distribution. If the process data is non-normal or it is the result of a combination of processes (a mixture of processes) then it provides an underestimation of the true non-conformance capability.