Appreciate help in understanding Sequential Test Method For Process Capability Decisions. It's claimed that this method can save upto 50% Sampling Savings. Examples welcome...

Gud times...

Hi Kiran,

If you look at the confidence interval of Cp/Cpk (process capability index), you will find that the confidence interval is pretty wide when the sample size is small. For example, the confidence interval is around +/- 0.4 when the sample size is 30. If you calculate the process capability index as 1.0, it could be as low as 0.6 or as high as 1.4.

If you use a smaller number of samples as recommended by the Sequential Test Method, say 15, then the confidence interval would be a lot higher. This would mean that the error in analysis could be a lot higher. A process that is shown to be capable may in fact be not capable.

So, I would recommend to use this method with caution.

Best Regards,

SJ

 

Dear SJ,

Request you to provide some more insight on sequential test method. What is it all about? in terms of its Application/benifits/ disadvantages.

Is it something similar in concept of using Frcational Factorial DOE rather than a full factorial DOE in Improve phase?

 

Dear Shalini,

This concept is very similar to Acceptance Sampling. Please refer to the following article: http://www.itl.nist.gov/div898/handbook/pmc/section2/pmc2.htm

Best Regards,

SJ.

Thanks SJ. will go through it and revert in case of further queries.

regds