Tuesday, October 30, 2007

Can you validate our Control Limit formula?

I follow your site regularly and find your thinking very refreshing. Thanks for sharing a wealth of knowledge with the community. I have a question bothering me for a long time,

While calculating Control limits in an SPC chart, I have seen several organizations use the following formula:

Lower Control Limit(LCL) , Upper Control Limit (UCL)

LCL = Mean - 3Sigma

UCL = Mean + 3Sigma
CL = Mean

What do you think?

What do I think? I'm thinking this gives me a headache!

I'm not a statistician, but I do write a blog, so of course that makes me an expert at everything! (I love the Internet!). Your formula is similar to the formula taught in the SEI's CMMI Understanding High Maturity Practices class and also in the best text I know on this subject, "Measuring the Software Process" by William A. Florac and Anita Carleton.

You'll need to read the book to gain a thorough understanding of the theory - but I warn you, save it for a night you can't sleep!


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