RE: Tom Ryan's reply to the SPC theory point

[Rip Stauffer <rstauffer@bluefirepartners.com>]

John Dowd has hit the nail on the head here, I think. One point that
Shewhart made very well was that, in the practice of industrial control,
process knowledge will out. Control limits are economic limits. He said we
are lucky to have mathematical probability models that are reasonably close
to the real world ("if we do not examine too critically what we mean by
'close'"), but if there is a conflict, we have to choose what the real world
gives us.

For anyone interested in experimenting, I have 50 sets of 50 pulls of red
beads taken under essentially the same conditions, an artifiact of my
master's thesis research. They are in an excel spreadsheet file. If you'd
like a data set like that to play with, let me know. Actually, I have two
sets. One is from a plastic bowl, plastic beads and plastic paddle, the
other from a plastic box with wooden beads and a metal paddle (and the
results were different).

John, where can we get a copy of the letter you reference?

Best regards to all,

Rip

Rip Stauffer, Senior Consultant
BlueFire Partners
1300 Fifth St. Towers, 150 So. Fifth St.
Minneapolis, MN 55402
612-344-1027
mailto:rstauffer@bluefirepartners.com
http://www.bluefirepartners.com/


In a letter he wrote to Deming dated Jan. 25, 1939, Shewhart discusses the
'bridge' between the physical state and the mathematical state and says,
"..the limits (are) not mathematical  but instead (are)  an empirically
chosen rule of operating" and that the 'bridge' between this 'practical' or
'physical' state of control is a 'long, long one in the sense that we must
have more than a thousand drawings before we can get over to the
mathematical statistician's universe or distribution function...."