Re: Using two different means and control limits on one control chart

[Paul Hollingworth <PH@paulholl.demon.co.uk>]

Certainly its OK, just so long as they represent explainable
process/system changes in different time periods -  don't change the
chart limits if you don't know why they have altered!
I no longer have my copy to hand, but Don Wheeler's 'Japanese Control
Chart' video used (and may still) to come with a 6 foot long example of
a chart with several different sets of limits. Every change is
explainable.
PH

hbendyk@srhs.com writes
>I teach a basic SPC class and a question arose
>that you may use two different means and two sets of control
>limits on the same SPC chart.  I was taught that if you
>make a change in a process you test the means.  If they are 
>different, you start a new control chart.  If they are the
>same, you may continue with the control chart as is.  
>I was told Deming supported the use of putting the two
>means and two sets of control limits on the same chart.  
>Is this true?
>
>Heather Bendyk
>hbendyk@srhs.com
>
>

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Paul Hollingworth                       4GM Consulting  
email: PH@4GM.com                       http://www.4GM.com
phone: +44(0) 1423 322225               fax: +44(0) 1423 322205 
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