I am just concerned about the "bright line" rule -- 1.33 and above = good. Below 1.33 = bad.We’ve answered this question many times now. The Cpk number is an estimate. ALL estimates are ‘wrong’. there is a confidence interval that will estimate the uncertainty in the estimate. If you have a small sample size then there is probably no practical difference. If you have a large sample size but you did not get a representative sample size then the estimate is wrong.
If you are concerned about the ‘predicted defect rate’ difference then that is just fake math and has no meaning whatsoever....
Only when people don't use their brains.Well, that darn line exists everywhere, right? Any spec where 1.33 is the minimum allowed, is 1.32 really not gonna work? How about 1.329? Maybe, maybe no one would notice, maybe it depends on other tolerance stack-ups whether it would or not.
In the case of a Cpk, I doubt it would matter from a practical standpoint, but practicality may not be the issue.
It always sucks to be near the edge of a spec.