From: Stan Hilliard
Date: 29 Aug 1999
Time: 19:46:19
Another way you could deal with too large of a StdDev is to use SPC to obfuscate the situation. Shewhart Xbar chart limits can be made to fit between the customer-proposed 5.475 to 5.525 specification interval by using a sample size of n=3, even though 1) the process is not capable and 2) control limits are different from lot acceptance plan limits.
5.0000+-3*.0112/Sqrt(3) = 5.0000+-0.025
Because of widespread misconceptions about SPC, this could deceive most customers -- deceived because the customer is not going to receive the quality of parts that he/she ordered.
It is a "smoke and mirror" approach because it deals with appearances only. I mention it because I believe that it is an everyday practice. It is the moral equivalent of the practices of the stereotyped used car salesman.
Sincerely, Stan Hilliard, CQE,CRE,CQA,PE shilliard@samplingplans.com