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  Statistical Techniques and 6 Sigma
  Statistically Valid Sampling Plans

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Author Topic:   Statistically Valid Sampling Plans
Forum Contributor

Posts: 245
From:St. Marys, PA
Registered: Sep 98

posted 02 October 1999 07:27 PM     Click Here to See the Profile for Dawn   Click Here to Email Dawn     Edit/Delete Message   Reply w/Quote
QS Auditor wants to see proof that 3 piece smaples for run charts is a statistically valid number. I have researched and pretty much proved he's right - its not. Employees feel five piece checks are too cumbersome. What to do? Thanks for any help-Dawn

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Posts: 111
From:Kane, PA 16735

posted 03 October 1999 11:00 AM     Click Here to See the Profile for Batman   Click Here to Email Batman     Edit/Delete Message   Reply w/Quote
Hi Dawn!
A subgroup of three may be right for you, five may be better, one may be correct also. Look at your process and determine the correct sample size and frequency. When we start a process, we take many samples in a short time, identify the assignable causes, remove or reduce them, start the process again, take less frequent samples, assess control, and so on, until the assignable causes are eliminated (or at least identified) and we know how long we can run until variation is expected to show up, thus samples are taken prior to this, for possible adjustment.

You can do this for present processes also, if needed.

I find that frequency has more to do with "control" than the sample size, except with multi-cavity tools.

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Don Winton
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Posts: 498
From:Tullahoma, TN

posted 03 October 1999 12:08 PM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote

Batman pretty much sums it up. Do not think of sample size in terms of the MIL-STD-105 mentality. Size and frequency are the keys. Show the assessor your justification in statistical terms where n=3 is valid.


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