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.

The...

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.

Since you are comparing two ranges – the range of output from the process to the range of the specs - it is natural to ask questions about how much these two ranges are different or overlap. This gave rise to numerical...