Re: SIX SIGMA and Shewhart's Maximun Control

[SOPKrules@aol.com]

Mikel Harry is referring to some sort of shift of the average due to tool 
life and other kinds of wear functions.

I do think that those who lead QS9000 and or six sigma iniatives are in good 
positions to bring into practice the tools that Dr. Shewhart and Dr. Deming 
used.  Regardless of whether you work with hard manufacturing or service 
jobs, the methods of statistical control are useful.   Cycletimes are most 
interesting and most enabling in terms of making grounded assessments about 
the time aspect of process performance.

Six sigma contains a base assumption of statisical control, and the base 
assumption of control is called into play with the use of the Capability 
Index.  The capability indeces allow you to assess your process' capability 
of meeting a specified range of variation that you can tolerate (tolerance 
range).  Cpk,  Cpu and others are measures of comparison between the limits 
specified by the design team up front (tolerance range) and the actual limits 
of variation shown on the control charts (natural range).  Essentially you 
have the Engineering Tolerance minus the Natural Tolerance divided by 6 sigma.

I believe the most significant characteristic of six sigma methodology is 
that the goal is to reach a level of capability equal to 2times the width of 
the Engineering Tolerances so that if your process was centered on the target 
and your upper and lower control limit (Natural Tolerance) were 2times as 
small as the Engineering Tolerance you would have Cpk of 2.0.   By related in 
the first sentence I mean the most useful with the practices Dr. Deming and 
Dr. Shewhart proposed.

By the way, Ford requires that you show minimum capability of 1.33.  The 
question is whether you do that in control or if you still have special 
causes.  What I think would be very interesting is some discourse and writing 
on the relationship between six sigma and Dr. Shewhart's writing on Maximum 
Control.  

Bob
The most practical significance is our usefulness of the thing.
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