CpK vs PpK

Patrick,

I believe Phil covered the general calculation of these two indices quite well. In his note, Phil referenced what he called the “overall variation.” Some also call this the “total variation” of the process or system. You can calculate the total variation by determining the standard deviation of all the individual values collected in the capability study. The total variation does not require the data to be time ordered.

The significance of Ppk vs. Cpk rests in the fact that Ppk compares the total process variation to the requirements, and does not require the data to be time ordered. Cpk compares the within subgroup, or inherent, variation to the requirements and does require the data to be time ordered.

Please see some of the other discussions on this site for additional details.

Ken

Hi Patrick
Dr Phil and Ken have given some very useful tips on the significance of Ppk & Cpk , I do not want to add to what has already been said ,but I would like you to go to the site below for getting a better understanding of what these indices are all about. The parent site is http://www.qualityamerica.com . I found this article very interesting. The link given below talks specifically of the significance of the Ppk index
http://www.qualityamerica.com/knowledgecente/knowctrInterpreting_a_Process_Capabilit.htm

Regards