Statistical Process Control (SPC): PPK versus CPK

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Statistical Process Control (SPC): PPK versus CPK

According to many, Six Sigma is based on Statistical Process Control (SPC), a method of visually monitoring production processes. In Six Sigma, each business process must be measured and for this often the SPC methodology is used. Aims of SPC are: 1. Adjustment: To detect quickly when a process needs readjustment or repair. 2. Improvement: To find the most urgently needed improvement in the system itself.

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		0 Alice, China
	PC What is the difference between PPK & CPK?

		0 Lo.Max, China
	What is the difference between PPK & CPK?" PPK is measured by unknowed process, cpk is measured by stable process

		1 Jacques-Alain TIAKO Manager, France
	Cpk or Ppk?? Cpk - It's the capability index for a stable process (the process must be in a state of statistical control). The sigma calculation is based on within subgroup variation. Ppk - It's the performance index (the actual process PERFORMANCE). The estimate of sigma is based on total variation. Ppk must be calculated if less than 100 samples or when the process is chronically unstable but meeting the specifications and in a predictable pattern.

		2 Ger de Waard Management Consultant, Netherlands
	What is the Difference Between PPK & CPK? In the Six Sigma quality methodology, process performance is reported to the organization as a sigma level. The higher the sigma level, the better the process is performing. Another way to report process capability and process performance is through the statistical measurements of Cp, Cpk, Pp, and Ppk. Definitions • Cp = Process Capability. A simple and straightforward indicator of process capability. • Cpk = Process Capability Index. Adjustment of Cp for the effect of non-centered distribution. • Pp = Process Performance. A simple and straightforward indicator of process performance. • Ppk = Process Performance Index. Adjustment of Pp for the effect of non-centered distribution. Cp and Cpk are for computing the index with respect to the subgrouping of your data (different shifts, machines, operators, etc.), while Pp and Ppk are for the whole process (no subgrouping).

		2 Ger de Waard Management Consultant, Netherlands
	Diff Between Cpk and Ppk @Alice: Both, the Cpk and Ppk are indexes for process capability. Cpk is for short-term variation, Ppk is used for long-term variation. Both compare the distance from the process center to the nearest specification limit and to the process spread. Cpk requires that the process be stable, while Ppk does not require process stability. So Cpk is effectively the best the process can perform when it is stable, and Ppk is the actual performance against that theoretical best.