Cp Cpk

## 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?

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:

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