Re: Shewhart's Maximum Control

[Lloyd Provost <lprovost@fc.net>]

"Mowery, R. Neal (RNM)" wrote:

> When I am trying to model cycle time, I try to match the skewed
> distribution.  But if I am trying to identify special causes of variation, I
> go ahead and use the control chart (usually XmR) as it does it's job of
> identifying when special causes enter the system regardless of the
> distribution.

Neal, are you familiar with any research on the ability of individual value
control charts for skewed data (like cycle time) to detect special causes? Don
Wheeler has done a lot of work on the affect of various distributions when the
system is stable, but I am not familiar with any papers that describe how the
skewness affects the X-charts ability to detect special causes.  Some of my
practical experiences indicate that an X-chart established for cycle times
without a transformation is not very sensitive when special causes occur.
Thanks.

Lloyd Provost
Associates in Process Improvement
512/451-0131
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