This week’s blog concludes our series on process capability and asks which is better: Cpk or Ppk? 

Your supplier has sent you the process capability chart you requested.  Looks like your supplier is really performing for you.  You note that his process has a Ppk = 1.14 and a Cpk = 2.07.  Why are those different?  Well, it doesn’t matter.  The Cpk is above 1.33, which is what you asked the supplier for.  You just missed a very important piece of information about your supplier’s performance.  Know what it is? 

The accompanying video explores Cpk and Ppk in more detail.

Our first two blogs in this process capability series answered two questions: What is process capability? – and - What is Cpk? This blog answers the next question: What is Ppk?

The answer is quite simple. Just to refresh your memory, Cpk is expressed as the following:

Cpk = Minimum (Cpu, Cpl)

Cpu=(USL-X)/3σ

Cpl=(X-LSL)/3σ

Our first blog in this process capability series explored the simple question “what is process capability?”.   There were two parts to the answer:

Process capability is the range of values that you can expect to get from the process over an extended time period.

A process is capable if the range of expected values fall within the specification limits.

You hear the term process capability a lot these days. Customers want to know your process capability. You want to know your supplier’s process capability.  So,  what is meant by the words process capability?  For a process:

Process capability is the range of values that you can expect to get from the process over an extended time period.