Six Sigma SPC - Statistical Process Control

Why Six Sigma And Not Seven Sigma?
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These articles are from the Six Sigma SPC Newsletter and other publications. All articles written by Jim Winings

From the (Feb-Mar-Apr) 2003 Newsletter


I was up on a six-sigma forum not too long ago. Now I was one of the first 100 people to go through six-sigma training in the world. A lot has changed, but not everything. Almost all of what I was taught can be traced back to Deming or Juran and that is still true today. As a matter of fact, most can be traced back to Deming or Juran. Some of what has changed works real well if youíre a large company, but is not cost efficient if you are a small company. Sometimes, some of the people at some of the larger companies donít understand this. But it is still a lot about common manufacturing statistics that have been around for years. Some people have found some short cuts to do things like calculate sigma, etc. but it is for the most part the same old stuff.

Things, like the basics, have not and should not change. For example, 1.5 sigma shift, (or hopefully lack thereof). I see people ask why not seven or eight sigma. Well, six sigma gives you 3.4ppm. (Parts Per Million) This is what Motorola decided they needed to have a 10 fold increase in profits. Why, because most of Motorolaís products are complex and have many steps in the manufacturing process. With many parts as well. It was based on a typical 1200 part step process. There is no magic to the 1.5 sigma shift as some may want you to believe. Juran indicated back in 1980 when referring to a 3 sigma wide bell shape curve on a normal distribution, that if your specification was at 4 sigma, then there was a better chance to catch bad parts before shipping them when you were doing SPC, (Statistical Process Control). While there is no reason why you could not use seven or eight sigma, it just wasnít practical from an economic point of view for Motorola to do that. At some point spending money to train and monitor people and processes just wasnít going to return on the investment. You CAN have too much of a good thing. Of course all of this goes ditto for GE, Honeywell, etc.

A lot of concepts of the original six sigma have been lost, as a lot of you well know. Lets face it, if you are just stamping out pieces of metal, how many things could possibly go wrong? Can enough go wrong to justify the training of a black belt? If you have a minority company of 10 people, how many hours could it take to eliminate a process problem? Who are the black belts going to train, and how many times? We need to keep six sigma practical by using common sense.

Back to the forum, one guy showed how you could estimate PI. You know, 3.14159. Now maybe he wants his autoís tires built with an estimate of PI, but I think I would like mine built using the real number. Just because something can be dome doesnít necessarily make it practical. And in some instances, it is just wrong.

Some of the people on this forum are the nastiest people I have ever run across. I canít believe they have had formal six-sigma training. One concept of six sigma is that there are no bad ideas. If you disagree with someone, you donít attack him or her personally. You attack their ideas maybe. They attack you personally. If this is what six sigma has come to then I want no part of it. They may know how to make the measurements and know the theory, but they are NOT getting it when it comes to the culture part. And six sigma, perhaps above all is about culture. A culture of common sense.


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Last Updated: Saturday, 15-Apr-06 19:47:35 PDT