Why are we still talking about Cpk and Ppk formulas?

Improvement Insights Blog

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? Why are why are you worried about formulas?” Your customer wants Cpk, and so all you have to do is go buy some software and let it calculate it. There’s three different ways to calculate Cpk depending on your sample size, right? Could be R Bar over D2, or S Bar over C4, or it could be pooled standard deviation… right? This is a complicated calculation. You’re not going to do it manually. I think companies out there hesitate to spend money to buy software to make it possible for their people to do stuff easily and effortlessly. Instead we have questions coming up about “Should I do R Bar?” No! Shut up. Stop. Stop worrying about formulas. Buy software that’ll do that for you.

“So that’s my Improvement Insight for this week: Get some software to make your life easy. Stop trying to do formulas. Don’t try and learn all that stuff; it’s already built into any package you buy.

“Let’s go out and improve something this week, and let’s do it quickly because we have some software that makes it possible!”