Hi Team,

 

In Green Belt, I learnt about Pp, Ppk Values for process capability.

 

I belongs to IT industry and my process improvement exercise requires me to calculate Cp, Cpk Value and based on the same decide whether the Process is Stable and Capable. Please guide me on :

 

- Is Cp,Cpk and Pp, Ppk values are same?

- What Cp, Cpk tells us?

 

Thanks...

Hello Panna

 

The concept of Cp and Cpk is similar to Pp and Ppk. However Pp and Ppk give us the actual performance of the process while Cp and Cpk give us the potential performance of the process. The difference is how we calculate the standard deviation. 

 

The decision criteria of process capability using Cp and Cpk is same as that for Pp and Ppk.

Hi Panna, 

 

Think of Pp and Ppk as the overall process performance - This performance assessment does not consider any sub-grouping. The formulae use the overall standard deviation, which has larger variation. This larger variation is due to the fact that between groups variation is bigger than within groups variation. Do remember that when we talk about grouping, it refers to logical subgrouping of continuous data in such a way that groups carry data that is homogenous.  

Think of Cp and Cpk as the potantial process capability - This performance assessment considers sub-grouping. The formulae use the within group standard deviation, which is lesser spread when compared to overall standard deviation. Cp and Cpk highlights capability that will be realised when between group differences are completely eliminated. Again, remember that we are refering to continuous data only when talking about these indices. 

When you go through Black Belt training, you will learn more nuances that relate to Process Capability assessment. 

Please note that none of these indicators relate to process stability. Stability assessment will require use of trend charts, run charts or control charts. 

Hope this helps. 

Ppk is used to assess the long-term, overall variability, whereas
Cpk is the capability index for short-term, potential variability.