Isn't the world of statistics wonderful. At some very large corporations (often automotive), Cpk is
equated almost to the bottom line. A Cpk simply provides some evidence of how well a process is
performing relative to whether it is in control (first) and whether it is capable of producing
product meeting specified requirements (second). This requires a processes' control limits to be
inside the product's specified acceptance limits. If you were to turn a Cpk chart on its side and
match its control limits to to those of an X bar R chart, or process control chart, you would get a
clear picture whether this was true.

The higher the Cpk's numeric value, the better is a process capable of performing. Also, remember
that when a process is said to be in control, this only means it is consistent. That is, it may be
effecting product either consistently good - or bad. When within the Cpk's specified limits, it is
said to be in control and effecting product meeting specified requirements. That's a very short
version of it.

Earl Moon

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