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  Statistical Techniques and 6 Sigma
  out of control patterns for non-normal dis.

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Author Topic:   out of control patterns for non-normal dis.
arezoo
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posted 12 June 2001 12:21 AM           Edit/Delete Message   Reply w/Quote
Hi
A process has a non-normal dis. and we prepare a control chart for that by using normal probablity plot.
Is there any "out of control patterns" for these control charts?
Maybe it is enough for these control charts only staying in their control limits ?
please help me to solve this problem.
thanks a lot
Arezoo

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Ken K.
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posted 18 June 2001 12:25 PM           Edit/Delete Message   Reply w/Quote
A control chart that uses averages does not necessarily need to assume the data to be normally distributed (thanks to the central limit theorem). As long as you are calculating averages for each subgroup AND have subgroup sample sizes of 4-5 or larger, you should do fine regardless of the distribution of the raw data.

There has been much debate as the the importance of normality in control charts. This is a no brainer for x-bar charts, not so obvious for some others. The R-chart is less robust against nonnormality.

On the other hand, nonnormality with individuals charts violates basic assumption of the method and be cause for concern.

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