Dear Friends,

Pp & Ppk are called long-term measures of process capability. Pp is called the Potential Process Performance Ratio and Ppk is called the Actual Process Performance Ratio. Together they are called overall capability indices.

Pp compares only the process spread with specification spread. However Ppk takes into account the relative position of process average w.r.t. the specification limits, in addition to comparing the process spread with Specification spread

A disadvantage of the Pp index is that it compares only the process spread with the specification spread, but not their locations.

Ppk is calculated using the overall process standard deviation. It measures the distance between the process average and the specification limits, compared to the process spread:

Pp & Ppk are useful in measuring capability over time. If your Pp / Ppk values differ greatly from your Cp / Cpk values, you can conclude that there is significant variation from one subgroup to another. i.e. your process is statistically out of control. Under such a situation, it is not advisable to use Cp, Cpk.