Specification Limits

Deming on Spec Limits vs. Variation

Deming contrasted US and Japanese focus: spec limits vs variation. Here’s the essence:

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

“I don’t know about you, but every once in a while I go back and read through books that I’ve gotten in Quality over the years. I went back into Deming’s “Out of the Crisis.” It was very interesting. There’s a thought he put in here and I’ll read it to you: “We in America have worried about specifications: meet the specification. In contrast, the Japanese are worried about uniformity, working for less and less variation around the nominal value…”

“What does he mean?

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Posted by Jay Arthur in Improvement Insights, Jay Arthur Blog.

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