Six Sigma
Six Sigma
(OP)
We're having a discussion about certain aspects of six sigma. Some texts include a chart that list sigma values for each yield percentage and include instructions on how to calculate yield percentage so you can look up your sigma performance level. The instructions don't indicate a test for normal distribution is needed. One faction contends that everything is "normal". Another says the books are wrong except for cases where the distribution is normal. A third faction chimes in with it doesn't matter if the distribution is normal, its just a convient way to compare results between projects.
We were wondering what other people thought?
We were wondering what other people thought?





RE: Six Sigma
Regards
RE: Six Sigma
RE: Six Sigma
RE: Six Sigma
The frequency distribution of many industrial quality characteristics is roughly normal. This is particularly so where the product comes from a single source within a short period of time. For this reason, the assumption of a normal distribution is good enough for practical purposes in many instances. This assumption is most likely a reasonable where inspection lots are formed close to the point of production, so that the chance for the mixing of product having different frequency distributions is held to a minimum.
Nevertheless, even though inspection lots have been produced under apparently homogenous conditions, it is always well to view the assumption of normality with a somewhat critical eye, investigating to see whether conditions exist that are likely to cause serious departure from a normal distribution. Sometimes the underlying frequency distribution is skewed, or it may be symmetrical but either peaked or flat-topped. The percentages in the extreme tails of such distributions may differ considerably from those obtaining under a normal distribution, and the protection against stated percentages of defectives given by the variables acceptance criteria may be either greater or less than the protection indicated by QC curves computed on the assumption of normality. The tighter the quality standards (for example the smaller of the AQL), the less reasonable it is to use the acceptance criteria based on the assumption of normality.
One important departure from normality exists when a producer has given 100% screening inspection by attributes to a lot prior to its variables sampling inspection by the customer. In such a case the frequency distribution in the screened lot may be truncated; one or both of the tails of the distribution may have been removed. With such truncated distributions, the variables criteria based on the assumption of normality may indicated that a lot should be rejected even though the actual nonnormal distribution in the lot may contain no defectives.
From “Statistical Quality Control”, Eugene L. Grant and Richard S. Leavenworth, 5th Edition , Page 527
http://www.alibris.com/search/search.cfm?chunk=25&mtype...
(Yeah, I know it’s old, but so am I. And I don’t think frequency distribution has changed too much in the intervening years…)
RE: Six Sigma
The purpose of the 3 , 6, 9 sigma is to determine the sample size and the frequency of taking samples. and the anticipation of production problems.
It means the ratio of tolerance given to machine capability.
You do not need equations, the normal distribution probability table can give you the answers you need.
RE: Six Sigma
"Many well educated and very intelligent engineers, scientists, and financial professionals with only brief introductions to statistics expect the normal distribution to fit everything. Six Sigma training does not always help here. Although other distributions, primarily the t, F, and chi-square distributions, are presented in basic Six Sigma training, many Six Sigma professionals feel that they have to force-fit the normal distribution to every data set."
Also see http://www.research.ge.com/cooltechnologies/pdf/2001crd...(starting about page 9) for an interesting list of some processes reported to be non-normal..
RE: Six Sigma
One of the main reasons for applying 6-sigma/statistical methods to our everyday world is to help increase our probability of making the correct decision.
It seems that lately there have been several discussions concerning cycle times on this board. Every mfg. process has a cycle time. This however, is a classic non-normal situation. Please see link below.
http://www.isixsigma.com/library/content/c020121a.asp
RE: Six Sigma
RE: Six Sigma
Statistical analysis needs a hands on approach. Just looking at inspection reports is unacceptable. Hands-on is required and understanding the process is a requirement.
Proper inspection equipment, calibration and use must be followed. Management must also repair equipment, tooling, and gaging when they are supposed to.
RE: Six Sigma
There isn't an original thought in all of their body of work. The intent was that CPK can be used to focus more attention in ways I can't begin to get in to here but they involve thinking about which side the mean and nominal fall into in relation to the specification limits and how repeatable the metrics are, especially since you have the same methods in industry applied to almost every specification, from stacked tolerances to you name it - I just said a lot if you think about it - which is what the Japanese intended. They actually sit around at work and talk to each other about their ideas. Imagine that!
RE: Six Sigma