Statistical Process Control (SPC): PPK versus CPK

Rating

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.

		Comments by date▼ Comments by rating Rating
		Alice, China
	0 PC What is the difference between PPK & CPK?

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

		Jacques-Alain TIAKO Manager, France
	1 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.

		Ger de Waard Management Consultant, Netherlands
	2 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).

		Ger de Waard Management Consultant, Netherlands
	2 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.

		Jaap de Jonge Editor, Netherlands
	1 The Relation Between Six Sigma and Statistical Process Control Six Sigma and Statistical Process Control (SPC) are both quality management tools used to improve processes and reduce variability, but they serve different roles and often complement each other. Six Sigma is a data-driven methodology focused on reducing defects and improving overall process quality. Its goal is to achieve near-perfect performance, typically defined as no more than 3.4 defects per million opportunities. Six Sigma uses the DMAIC framework (Define, Measure, Analyze, Improve, Control) to drive continuous improvement. Statistical Process Control (SPC), on the other hand, is a set of statistical methods used to monitor and control a process. It primarily involves the use of control charts to detect process variation and ensure the process remains stable over time. The relationship between the two is synergistic: SPC is a tool within the Six Sigma toolkit. It helps measure and monitor process performance during the "Control" phase of DMAIC. Six Sigma uses SPC to differentiate between common cause and special cause variation, enabling more effective problem-solving and process improvements. SPC provides real-time feedback, allowing Six Sigma practitioners to maintain gains after improvements are implemented. In short, while Six Sigma provides the overall approach for process improvement, SPC is a tactical tool used within that approach to maintain control and monitor progress. Together, they drive sustained quality and performance excellence.