Cp Cpk

Capability Analysis Issues

Struggling to get a good Cp and Cpk? It might be your data. Here’s why:

“I’m Jay Arthur, author of “Lean Six Sigma Demystified” and QI Macros [software].

“We had a guy call in the other day and he was trying to determine the capability of his process. Unfortunately, he’d taken [measurements of] everything from his startup to his shutdown scrap and ran that as a capability study, and he wasn’t capable. I said, “Well, there you go…” But if you run a control chart of that, you can see the wiggly startup and the wiggly shutdown and then there’s this nice stable process in the middle.

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Posted by Jay Arthur in Improvement Insights.

Why Are We Still Talking about Cp/Cpk and Pp/Ppk Formulas?

I found a discussion on MyASQ’s Website about Cpk formulas. Why is anyone talking about how to calculate formulas by hand? It’s a waste of time. Here’s why:

“Hi, I’m Jay Arthur, author of “Lean Six Sigma Demystified” and QI Macros [software].

“This morning I was out on the myASQ website, and there was a question about Cp and Cpk and Ppk formulas and stuff like that. And there was, you know, “Well, standard deviation” and “Why do we have to do R Bar over D2?” or whatever it was, you know. The answers were like, “Well, you know, standard deviation is pretty close to whatever…”

“I was [thinking], “What is the question here?

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Posted by Jay Arthur in Improvement Insights, Jay Arthur Blog, QI Macros, Six Sigma, Statistics.

Who Sets Your Specification Limits?

A QI Macros customer called to ask how he could figure out the specification limits to give him a Cp/Cpk of 1.33. What’s wrong with this question? The answer is obvious isn’t it?

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Posted by Jay Arthur in Improvement Insights, QI Macros, Six Sigma.

What USL/LSL Do I Need for Cp>1?

Customer asked me what seemed like a strange question: What specification limits do I need to get a Cp greater than one? Usually her customer should set specification limits, but her boss wanted to know what they could deliver. Hmmmm!

Then I realized that since QI Macros templates (e.g., XmR chart) calculate the average and sigma estimator, the LSL/USL for Cp = 1.0 would be:

LSL = Average – 3*SigEst USL = Average+ 3*SigEst

For Cp = 1.33, just change the 3 to a 4; Cp = 1.66, change the 3 to a 5. Here’s an XmR chart template with some sample data and calculations to reverse engineer spec limits:

Cp-to-LSL-USL