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We did a capability study on the straightness of some shafts. Our sample size was 200. We have a Cp value that is less than the Cpk.

Why does this happen?

I already know that the process is not capable. That is why we are doing the study, to find out just how uncapable it is.

We had two points out of tolerance and one on the dead high, the Cp was .95 and the Cpk was 1.00.

I haven't been able to find anything in any of my SPC books that explain what causes this.

BTW Marc,

I just wanted to say thanks for providing this site. And thanks to all that contribute to it. It has really helped me a lot.

Thanks