J
I am going to bring back an old topic discussed here around 99. When to use Ppk/Pp and Cpk/Cp. someone then wrote
"Speaking strictly as someone trained (somewhat) in statistics, I have never, before the QS thing, thought that Ppk was required, nor necessary (never heard of it, actually). Ppk is something for those who do not understand the difference between population and sample. The unbiased estimate of sigma should be more than enough."
Well in my industry-I believe I am an exception, and let me explain why. We are a chemical process industry that sends large batch runs into tanks. These tanks are kept homogenous by stirring and then sampled.
Therefore, the sample's standard deviation is the sigma of the population.
I allege that we must use Ppk/Pp because we don't need to estimate population standard deviation -- We already have it. It is sigma of the individuals and so we should use Ppk/Pp. my only complaint with this is that most software packages use the degree of freedom correction for standard deviation, when none is warrented.
why is this relevant? Another analyst has been doing a GMP validation study and making these sweeping conclusions based upon Cpk/Cp. what do the guru's of the cove say??
"Speaking strictly as someone trained (somewhat) in statistics, I have never, before the QS thing, thought that Ppk was required, nor necessary (never heard of it, actually). Ppk is something for those who do not understand the difference between population and sample. The unbiased estimate of sigma should be more than enough."
Well in my industry-I believe I am an exception, and let me explain why. We are a chemical process industry that sends large batch runs into tanks. These tanks are kept homogenous by stirring and then sampled.
Therefore, the sample's standard deviation is the sigma of the population.
I allege that we must use Ppk/Pp because we don't need to estimate population standard deviation -- We already have it. It is sigma of the individuals and so we should use Ppk/Pp. my only complaint with this is that most software packages use the degree of freedom correction for standard deviation, when none is warrented.
why is this relevant? Another analyst has been doing a GMP validation study and making these sweeping conclusions based upon Cpk/Cp. what do the guru's of the cove say??