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  Statistical Techniques and 6 Sigma
  Non-normal Dis. in SPC

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Author Topic:   Non-normal Dis. in SPC
posted 19 May 2001 06:46 AM           Edit/Delete Message   Reply w/Quote
Hi everybody
I want to impelement SPC in our company.
there are some process that they don't have Normal distribution.Is there any SPC method for Non-normal Dis.?please help me to solve this problem

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From:Tehran - IRAN
Registered: May 2001

posted 19 May 2001 11:50 AM     Click Here to See the Profile for MSAFAI   Click Here to Email MSAFAI     Edit/Delete Message   Reply w/Quote
Dear Arezoo,

Since I'm not a specialist in this field, I can only give you a hint:

One of the methods to deal with non-normal distributions, is to use data 'transformation'. Meaning, depending on the distribution shape, you use a formula to transform the distribution to a normal one. For example in some cases you can use the square root of the x.

Please have a look at Juran's quality handbook (5th edition) for the outline and some references.

Good Luck

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Ken K.
posted 20 May 2001 12:52 AM           Edit/Delete Message   Reply w/Quote
Along those lines, both MINITAB and JMP have Box-Cox transformation tools that allow you to easily identify a transformation that provides normality.

MINITAB also gives a confidence interval for the tranformation constant, which means you have the choice of a range of "best" tranformations. If 0 is inside that CI, then you'd just take the log of the data. If 0.5 is inside, you'd take the square root, etc...

You will also run into a lot of people who say don't sweat the nonnormality, but haven't seen anything to justify that lack of concern.

On the other hand, if you are taking subsamples that are larger, say more than 5-10 or more, then you can take advantage of the fact that the sample mean, regarless of the parent population's distribution, WILL follow a normal distribution (this is what the Central Limit Theorem is about).

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