Looking for some information about sample sizes to be used during audits for verification of process conformity. (the processes I have are not manufacturing processes - we are a distributor. sample sizes here are taken of material locations to verify our bin counts.) I am aware of MIL-STD-105 & ANSI/ASQC Z1.4 - 1993. Neither of these are specific to auditing as far as I can tell. Does anyone have any ideas?
Thanks Elaine
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firebrew 9/18/2003
Both Cp and Cpk are estimates of expected process capability. As estimates of something that can be reasonably expected, they are predictions.
Cp refers to the ratio of "natural" process variation, to the width of the specification limits. The natural variation range (6 sigma) is divided into the specification range.
Cpk refers to a similar estimate, but substitutes the distance between the "natural" variation mean and the closest specification limit, for the specification range.
Because these indeces are predictions, they both require some type of control chart to demonstrate that the natural variation range is statistically stable.
Many people attempt to estimate Cp or Cpk from histogram type data without first establishing statistical stability of the process being estimated. This should never be done. Something called Ppk can be used to characterize the observed capability using such data.
trimarchi 9/15/2003
d2 parameter to calculate sigma starting from moving range
d2 is defind as function of sample size: d2=d2(n). I only found values for d2 up to 25.
what is the meaning of sample size.
I will give you an example:
I have 10 pieces with 15 measures each (150 sample at all). what is the value of n I have to refer to (10,15,150)?
many thanks