Q 79. For Process Performance Assessment, one may use Capability / Performance Indices (Cp, Cpk, Pp, Ppk) or Sigma Level inter-changeably. Compare and contrast their utility and highlight why someone would prefer one over the other. 

 

Note for website visitors - Two questions are asked every week on this platform. One on Tuesday and the other on Friday.

• All questions so far can be seen here - https://www.benchmarksixsigma.com/forum/lean-six-sigma-business-excellence-questions/
• Please visit the forum home page at https://www.benchmarksixsigma.com/forum/ to respond to the latest question open till the next Tuesday/ Friday evening as per Indian Standard Time.  
• The best answer is always shown at the top among responses and the author finds honorable mention in our Business Excellence dictionary at https://www.benchmarksixsigma.com/forum/business-excellence-dictionary-glossary/ along with the related term.

 

I believe that the excellence ambassadors will be familiar with the fundamental definitions and calculations for Cpk, Ppk and Sigma. So, I am not elaborating on it and straight away getting into the comparative discussion.

 

Process capability index Cp is calculated using the variations within a rational sub-group and hence indicates the process potential, but does not include the likely real time variations between groups, which could influence in the long term.

 

Cp is a very good measure to be used for assessing the inherent process potential. It will be useful to assess the impact of any change / improvement on the process capability for a given process.

 

Process performance index Pp considers the actual overall variation for its calculation and hence gives a more realistic prediction of the process performance.

Although the Cp may appear good (say >1.67), it is important that Pp is also calculated to assess the process performance over time when subjected to day to day variations of real life production. Pp will serve as a measure for Production part approval criteria.

 

When the above measures are done in consideration with the center shift of the mean, assuming we have an upper and lower specification limits, the respective measures Cpk and Ppk are used.

Unless the study is used for a very short run, it is always recommended to use Cpk or Ppk as the case may be.

 

When the process variation is statistical control, Cp and Pp tend to become equal.

Cpk and Ppk are measures that are meaningful only when we have an upper and lower specification limits, and ideal while we deal with variable data.

 

When we need to express the process performance index comparable across different processes that would also include attribute data, the sigma level as a measure of process performance becomes useful. Since we there is an established relationship between the defect levels (DPMO), and the associated sigma levels, it becomes a versatile measure to express and compare process performances. However, it is recommended to maintain the Cpk and Ppk values for the benefits discussed, and maintain the corresponding sigma conversion for having company-wide uniformity.

	Cp	Cpk	Pp	Ppk
Definition	Process Capability	rocess Capability Index	Process Performance	Process Performance Index
Indicates / Implies / Features	A simple number indicates process capabilities.   Gives the best case scenario whether the process is capable of fitting into the specs are not.   Should be greater than > 2.0	How close we are to the taget, or how close a process is running compare to given specification   The higher the index, the lower the chance to get outside of the specs   what a process is capable of doing in future – assuming the rest control state is static.   Gives the best case scenario whether the process is capable of fitting into the specs are not.   Should be greater than > 1.5	A simple number indicates process performance   when process control can not be determined, Pp is used.   It uses procerss sigma for calculation.	Whether the process will meet customer requirements
Used for		Short term		Long term
Calculation	Cpl = (Mean – LSL)/3*S   Cpu= (USL – Mean)/3*S	Cpk= Min (Cpl,Cpu)	Pp = (USL – LSL)/6*S PpI = (Mean – LSL)/3*S PpU = (USL - Mean)/3*S	Ppk= Min (Ppl,Ppu)