Customers expect a consistent quality product that meets their specifications as it is produced - not by sorting or rework. 


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.


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.


If the individual measurements are normally distributed, nearly all the data (99.7%) will fall between the average and +/- three standard deviations (s) of the average.


The natural tolerance is the distance from -3s to +3s. 


If the specifications or standards are outside the distribution, the process is meeting specifications. 


The upper specification limit (USL) minus the lower specification limit (LSL) is called the engineering tolerance (ET).


One capability ratio, Cp, is defined as the ratio of the engineering tolerance to the natural tolerance:


Cp = ET/NT = (USL - LSL)/(6s)


If the engineering tolerance is less than the natural tolerance (i.e., Cp < 1.0), the process is not capable of meeting specifications. 


If the engineering tolerance is equal to or larger than the natural tolerance (Cp > 1.0), the process is capable of meeting specifications. 


A process can be capable of meeting specifications but not be meeting specifications if the process is not centered relative to the specifications.

“Process Performance Index basically tries to verify if the sample that you have generated from the process is capable to meet Customer requirements. 


It differs from Process Capability in that Process Performance only applies to a specific batch of material. 

Samples from the batch may need to be quite large to be representative of the variation in the batch. 


Process capability indicates whether a process potentially can meet a specification.

 

Process performance indicates how the actual process performed over a period of time.


Process Performance is only used when process control cannot be evaluated. 

An example of this is for a short pre-production run. 


Process Performance generally uses sample sigma in its calculation; 

Process capability uses the process sigma value determined from either the Moving Range, Range or Sigma control charts.”


“Pp = (USL – LSL)/6*Std.dev

Cpl = (Mean – LSL)/3*Std.dev

Cpu = (USL – Mean)/3*Std.dev

Cpk= Min (Cpl, Cpu)”