Any One Could Tell Me When I Should Use Cpk or Ppk? Thks!

Preston,

In fact there are 4 such measures : Cp, Cpk, Pp & Ppk. The first 2 are for computing the index with respect to the subgrouping of your data (different shifts, machines, operators, etc.), while the last 2 are for the whole process (no subgrouping). For both Ppk & Cpk the "k" stands for "centralizing facteur"- it assumes the index takes into consideration the fact that your data is maybe not centered (and hence, your index shall be smaller).

It is more realistic to use Pp & Ppk than Cp or Cpk as the process variation cannot be tempered with by inappropirate subgrouping. However, Cp and Cpk can be very useful in order to know if, under the best conditions, the process is capable of fitting into the specs or not.It basically gives you the best case scenario for the existing process.

I hope this helped you a bit. If you have anymore questions, don't hesitate to contact me.

Chantal