What Is A Stable Process?

Process stability is one of the most important concepts of the Six Sigma methodology, or any quality improvement methodology for that matter. Stability involves achieving consistent and, ultimately, higher process yields through the application of an improvement methodology. Does a process need to be stable in order for a black belt to improve it? Try educating your child when he/she is having fits of joy and emotional distress at the same time (as many youngsters do!).

To help your understand why process stability (also known as variation reduction) is important, here are a few illustrative concepts:

Building widgets to be within customer specifications or tolerances: Some variation is inherent in the process, but the variation may not be wider than the specifications allow, otherwise the widget may not fit or function properly.

Mailing welcome packages after signing up for an online trading account: Customers expect time delay in receiving welcome materials due to processing, printing, the postal service, etc. But customers have specified that they expect receipt within 10 business days -- anything over indicates the process is unstable and the variation is too large. As an aside, anything over may also cause additional customer telephone calls to the processing center, which can add to customer service costs.

Changing the oil in your car is important every 3000 miles (or now even longer): Adding too much or too little oil may cause malfunctioning of your engine under certain circumstances. Again, variation is inherent, but the specifications of high or low oil must be met.

A loyal reader started a fabulous discussion on process stability (thanks Kim!), and I thought it would be useful to present some of the finer points. Below are a few posts from fellow quality professionals discussing how best to link quality to finances. If you have a question or would like to make an additional comment, just press the 'Post A Reply' button.

"Montgomery states in his book: Montgomery, Douglas. C. 'Introduction to Statistical Quality Control'. Wiley & Sons, Inc. New York. 2001. 4th ed. Pg 372, that in 1991 the Automotive Industry Action Group (AIAG) was formed with one of their objectives being to standardize industry reporting requirements. He says that they recommend Cpk when the process is in control and Ppk when it isn't. Montgomery goes on to get really personal and emotional about this which is unique to this page of this book and other books I have of his. He thinks Ppk is baloney as he states 'Ppk is actually more than a step backwards. They are a waste of engineering and management effort - they tell you nothing

"While Montgomery gets frustrated over the use of Ppk, he does a poor job of explaining what a stable process is. I respect his works just the same as he alone has even attempted to try to explain the difference between a stable process and a non-stable one.

"This argument as well as numerous others that you can find on this site regarding use of Cpk / Ppk metrics, the validity of Six Sigma shifts, process capability, SPC, etc. all reflect our lack of definition for what is process control.

"So, how can we define process stability and or process control? Perhaps we can agree on some given amount of process shifting (1.5 sigma)? Perhaps we can agree that a stable process is that where it's Cpk values are above 1.67? Perhaps some combination of these or other events needs to take place such as three consecutive Cpk samples over 1.67, etc.