kbower50

I was not advocating the use of Pp and Ppk.  In particular, I am in agreement with Montgomery’s discussion on this in his most recent SQC book.  Merely the terminology was addressed.  Of course, with a stable process, Pp and Cp should be very similar, as should Cpk with Ppk.  Since many quality practitioners I meet use these indices such a dis…[Read more]

I just wanted to clarify one section of your answer as it may be misinterpreted from first inspection:
In particular,
Many people will tell you that the data needs to be normally distributed. That is simply not true. Cp is a just ratio–nothing more. Once you use Cp (or Cpk) to estimate percent defective then you must find if the data is truly…[Read more]

Minor correction, if we are to use the Automotive Industy Action Group (AIAG) terminology.  Cp uses the within-subgroup (a.k.a. short-term) standard deviation, hence we require Cp = Cpk = 2 for a “six sigma” process.  Hence, in the “short-term” (theoretically, of course) we’re actually at .001ppm. 
When the long-term shift of 1.5 sigma is ta…[Read more]

Sir R.A. Fisher’s original work on DOE was in agricultural experimentation, so the experimenter will be in fine company.  I would urge you to look at effects including blocking (piglets nested under each pig) for optimum feeds, etc.  I’d refer you to Montgomery’s DOE books (e.g. 2000) for a fuller, contemporary discussion on DOE, as well as Box, H…[Read more]

You’re right – it used to be called the homogeneity of variance test, but was changed to “test for equal variance” due to customer demand.  I’ve a paper on using MINITAB to do ANOVA, along with relevant references, reproduced from SC&I at http://www.minitab.com/company/VirtualPressRoom/Articles/SCIFeb2000ANOVA.htm
Hope it helps.  There’s also a p…[Read more]

The 2nd law of thermodynamics provides an incentive to give ourselves some margin of error when thinking of how a process will behave over time.  The use of 1.5sigma as a value throughout all industries is controversial.  If the reader is pragmatic, however, at least we’re moving away from the unrealistic viewpoint of a process fluctuating ra…[Read more]

Hello, I’m very concerned with your statement that you can report capability indices for a non-stable process.  This would not be good practice if one were interested in reporting indices to suggest to a customer how your process will perform over time.  If we have unreliable parameter estimates for the process mean and process standard de…[Read more]

I have a tough time with the concept of 1.5 sigma being the shift.  It may apply to a particular company at a particular time, but not for all companies.  However, I feel that one has to be pragmatic.  The second law of thermodynamics is a pain, but it exists.  If everybody uses the same sigma shift (1.5) then at least everybody is speaking the…[Read more]

Other good sources of information are Montgomery’s most recent SQC book and there are a couple of papers available from the Minitab website through the Company>Virtual Pressroom>Magazine Articles and Reprints route.
There will also be a paper on the use of confidence intervals for Cp and Cpk at that site shortly, the macro of which may already be…[Read more]

To reinforce the previous note, if you use the macro available from the Minitab website, using the information you provided, an approx 95% CI for Cp is 1.51 to 9.96.  For Cpk, an approx 95% CI is 1.06 to 9.66.
I’m using the usual caveats of Normality and stability for this type of analysis to even make sense in the first place, of course.

If there is a significant interaction effect, the Xbar-R computations give lousy estimates of the reproducibility aspect.  Using ANOVA we have ways around it (e.g. dropping the interaction term and refitting the model, as Minitab does if the p-value is >0.25 for the op*part effect.)  You may want to check out my gage r&r paper at h…[Read more]

The arguments for the robustness to Normality are associated with Tchebycheff’s inequality, as is discussed in Shewhart’s original discussions on control charting.  It’s true that the CLT will allow for approximate Normality, hence the false alarm rates frequently referred to would be valid in such an instance (importantly, related to…[Read more]

Matt, check out my second paper on capability analysis – it deals with non-Normality.  It can be found on the Minitab website via Company>Virtual Pressroom>Magazine Articles and Reprints.  There’s a discussion on Box-Cox transformations as well as Weibull modeling.  You can download the data I used for the examples in the paper from the Minitab si…[Read more]