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This thread is carried over and continued in the Current Elsmar Cove Forums
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The New Elsmar Cove Forums
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 The New Elsmar Cove Forums
 Statistical Techniques and 6 Sigma
 Non-normal Dis. in SPC
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| Author | Topic: Non-normal Dis. in SPC |
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arezoo unregistered |
posted 19 May 2001 06:46 AM
 
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 IP: Logged |
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MSAFAI Forum Contributor Posts: 24 |
posted 19 May 2001 11:50 AM
 
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 IP: Logged |
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Ken K. unregistered |
posted 20 May 2001 12:52 AM
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). IP: Logged |
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