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"Normal" vs. "Between/Within" Capability Analysis

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steph4444

#1
Dear all,

For my first post, I would like to address the issue of "normal" versus "between/within" capability study.

It all boils down to a) the presence of subgroups and b) the calculation of standard deviation

In an aeronautical context, I need to assess a process performance based on sample batches (3/4-samples) extracted from structural parts, hence I consider each part as coming from a different process stream. I also consider each sample batch as a subgroup.

The current method uses the minimum of 2 indices, PpK(ind) based on SD of individual values, and PpK(avg) based on SD of averages of each subgroup, to determine performance.

Here are my questions :

Does PpK(avg) make sense ? (SD calculated straightforwardly with (n-1))

Shouldn't one use "between/within" capability study ? (variation between subgroups is important)

How do you justify this ? And more importantly, how do you conduct it ? I now have several SD : "within", "between", and "between/within"

I was thinking of computing PpKs using "between/within" instead of "overall" SD, which is twice bigger, and also "between" SD to show variability in-between process streams...


I cannot find much documentation on the web on "between/within capability study", but you folks seem to have all the savvy needed, so please feel free to drop your 2 cents worth, Ill very much appreciate it.
 
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#2
Re: Between/Within Capability analysis

Shouldn't one use "between/within" capability study ? (variation between subgroups is important)
It the case of multiple streams, my preference has always been to provide capability of each stream. Typically it is multi-modal enough, but combining them makes them so multi-modal that normality is totally out of the question and the statistics of such multi-modal distributions does not represent anything meaningful.
 
S

steph4444

#3
Re: Between/Within Capability analysis

Maybe I am misguided in the use of the term "process stream". I meant that two produced parts are released within a rather large time-frame (week/month). Hence common causes could be greater and also, process may seem to vary since parts may not be produced with the exact same machine tools, operator etc...
But essentially it can be considered as the same process.

Normality is verified on both individual values and subgroup averages...
 
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