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...