RE: The rationale for substantive knowledge of underlying process

[Kromkowski@aol.com]

<< The XmR chart is also  not the chart of choice since the Average/Range 
chart is more sensitive to  process upsets using similar detection rules 
ASSUMING THE SUBGROUPING AND SAMPLING FREQUENCY makes sense for that process 
(process knowledge required?) >>

Here is a very interesting question.  I know well that X-BAR chart is the 
chart of choice.  Why?  Because it will signal more special causes? In other 
words, is it really chosen because it keeps people occupied with something 
other than the hard issues of systemic causes of variation?  Who knows?

<< If the subgrouping is incorrect, it will NOT lead to a special cause but a 
false alarm or missed signal, just the thing Shewhart was trying to 
minimize.>>

I disagree, I suppose we could do some simulations.  There may be an 
operational definition issue.  If there is a signal of special causes, is not 
"Did I use the "appropriate chart/is the subgrouping rational" an appropriate 
line of inquiry?

>>  An educated 7th grader educated in what? My 8th grade daughter is
 educated but how would she know how to set up a control chart for a
 plasma-etch process in a GaAs wafer fab? Do you think a little understanding
 of the process would be helpful? Maybe a little knowledge about metrology,
 sampling, calibration, measurement method/technique?  Nah....just throw it
 on a XmR chart and interpret.  >>

Educated in the choice and construction and interpretation of control charts. 
Read or reread Plato's Meno dialogue in relation to your daughter.  I have 
little doubt that given the _available_ data she could construct and 
interpret a control chart with pencil, graph paper, ruler, and calculator.  
Calibration and measurement methods and techniques are sources of variation 
(in some cases special and in other cases common and often both).  

As to my suggestion that the separation of common from special causes is not 
all that complicated: I stand by it.  The tough part is doing something about 
them, at which point substantive knowledge is required.  There are many 
reasons why people want to take knowledge and shroud it in secrecy and 
expertise and complexity.