Eliud Kipchoge
Registered
Hello,
Excuse me for asking, but I have an issue that is driving me crazy. I have a single batch, with 110k datapoints on it. These datapoints are automatically recorded and have both good and bad products. Bad products get discarded inmediately afterwards, and therefore, do not continue in the process.
We usually calculate process capability parameters for each batch, filtering only for the good products, using the reasoning that a bad product is 'only' a yield problem. Is this approach correct?
To calculate the capability, we use Cp and Cpk. It is only one batch, but with over 100k datapoints (around 9k are gone after removing the bad products). I understand that Cp and Cpk both use sigma (in other words, population standard deviation, or the one with N in the denominator). I also know that Pp and Ppk (which we usually do not calculate) use s (i.e, sample standard deviation, the one with N-1 in the denominator).
So far, for each batch, we have always used Cp and Cpk. Therefore, we calculated the parameters with sigma (also known as OVERALL on Minitab software). Is it acceptable to use Cp and Cpk in our case (for single batch, but with a lot of datapoints) or should we use Pp and Ppk for each individually batch (and Cp/cpk when?). If Pp and Ppk, what would be my subgroup size?
There is a final twist (that is the bane of my existance...). The machine prints out a report for the batch. This report states both specification limits, mean of the datapoints, standard deviation, RSD% (relative standard deviation %) and Cp and Cpk (named exactly like this, Cp and Cpk, not Pp(k)). Specs limits and mean are ok. On "standard deviation" they calculate sigma (population), which I agree with (let's suppose the value is 4).
But for RSD% calculation, they do not use 4 (population std dev), they use 3.26, which is the value Minitab calculates for sample standard deviation (taking subgroup size as 1, which I do not understand...). And for Cp and Cpk they also use again the sample standard deviation. If Pp and Ppk should be calculated for this, then the report should show s and not sigma. But if Cp and Cpk should be calculated, then the parameters are plain wrong! Which situation is it?
Sorry for the sea of doubts. Any help or insights would be highly appreciated. Thanks a lot!!
Excuse me for asking, but I have an issue that is driving me crazy. I have a single batch, with 110k datapoints on it. These datapoints are automatically recorded and have both good and bad products. Bad products get discarded inmediately afterwards, and therefore, do not continue in the process.
We usually calculate process capability parameters for each batch, filtering only for the good products, using the reasoning that a bad product is 'only' a yield problem. Is this approach correct?
To calculate the capability, we use Cp and Cpk. It is only one batch, but with over 100k datapoints (around 9k are gone after removing the bad products). I understand that Cp and Cpk both use sigma (in other words, population standard deviation, or the one with N in the denominator). I also know that Pp and Ppk (which we usually do not calculate) use s (i.e, sample standard deviation, the one with N-1 in the denominator).
So far, for each batch, we have always used Cp and Cpk. Therefore, we calculated the parameters with sigma (also known as OVERALL on Minitab software). Is it acceptable to use Cp and Cpk in our case (for single batch, but with a lot of datapoints) or should we use Pp and Ppk for each individually batch (and Cp/cpk when?). If Pp and Ppk, what would be my subgroup size?
There is a final twist (that is the bane of my existance...). The machine prints out a report for the batch. This report states both specification limits, mean of the datapoints, standard deviation, RSD% (relative standard deviation %) and Cp and Cpk (named exactly like this, Cp and Cpk, not Pp(k)). Specs limits and mean are ok. On "standard deviation" they calculate sigma (population), which I agree with (let's suppose the value is 4).
But for RSD% calculation, they do not use 4 (population std dev), they use 3.26, which is the value Minitab calculates for sample standard deviation (taking subgroup size as 1, which I do not understand...). And for Cp and Cpk they also use again the sample standard deviation. If Pp and Ppk should be calculated for this, then the report should show s and not sigma. But if Cp and Cpk should be calculated, then the parameters are plain wrong! Which situation is it?
Sorry for the sea of doubts. Any help or insights would be highly appreciated. Thanks a lot!!