
This thread is carried over and continued in the Current Elsmar Cove Forums 
The New Elsmar Cove Forums 
The New Elsmar Cove Forums
Statistical Techniques and 6 Sigma Nonnormal Dis. in SPC

UBBFriend: Email This Page to Someone!  next newest topic  next oldest topic 
Author  Topic: Nonnormal Dis. in SPC 
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 Nonnormal Dis.?please help me to solve this problem IP: Logged 
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 nonnormal 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 
Ken K. unregistered 
posted 20 May 2001 12:52 AM
Along those lines, both MINITAB and JMP have BoxCox 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 510 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 
All times are Eastern Standard Time (USA)  next newest topic  next oldest topic 
Hop to: 
Your Input Into These Forums Is Appreciated! Thanks!