Quality Blog


Quality Remix: Six Problems I have with Six Sigma, Part I

November 23, 2009
/ Print / Reprints /
ShareMore
/ Text Size+
I have big problems with the Six Sigma approach, and would not recommend it to anyone. Let me tell you why. This blog will take place in six parts, where I will tell you the six problems I have with Six Sigma.

The popularity of the Six Sigma program is truly amazing. An idea that started by extending the routine process capability study from plus and minus four sigma to six sigma has spawned a miniature industry. Software, training, books, lectures, magazine article, consulting, advertising, certification and on and on. An entire methodology has developed; it is truly astonishing. Some people are making a lot of good living practicing Six Sigma, however I seriously doubt if many defects have been prevented.

It’s no secret that I don’t care for the Six Sigma idea. If you read my articles or books (my latest is The Zero Defects Option), I’m up front about it. However, if Six Sigma is what you want to do, it’s your option. As Nike says: “Just Do It.” If it pleases your boss or your customer, it would be foolish not to do it to do it.

I have big problems with the Six Sigma approach, and would not recommend it to anyone. Let me tell you why. If I’m wrong, maybe some Six Sigma belt-person will help me understand where I’m going wrong. This blog will take place in six parts, where I will tell you the six problems I have with Six Sigma.

Problem one, The Concept


I see Six Sigma as a performance standard-which is 3.4 defects per million opportunities (DPMO). I guess it means that 3.4 defect/million is okay-or maybe not. Sigma, of course, is a unit of distance of individuals from the average of a population. However, I don’t see where Six Sigma has much to do with sigma. This is my first problem with Six Sigma. Very few people understand sigma-standard deviation-or can calculate it, or even care. So, Six Sigma must be translated into everyday language: 3.4 defects per million operations, or DPMO as they say. I’m not sure what four tenth of a defect is, but that’s okay.

How does one calculate sigma? One Six Sigma book I read used a kind of conversion table showing % yield and sigma values in two columns. If you follow the yield column until you reach 99.99966%, that’s 3.4 DPMO, or 6 sigma. That’s probably why a black belt once told me that it was simple to calculate sigma; you simply multiply by 3.4. Really? She wasn’t clear about what to multiply by what.

As any quality professional can tell you, there is sigma and there is sigma. It looks to me like sigma used in Six Sigma is only an estimate of standard deviation; sometimes called sigma hat. In case you don’t know-in a process capability study-sigma is calculated by dividing the average range of the samples by a constant value (d2), which taken from a statistical table. So, it’s an estimate because the individuals measured are just samples of the process. Every characteristic of every part could have a different average range, thus a different sigma. If you make thousands of parts with thousands of characteristics, you would have that many sigma values. Which sigma value is used for Six Sigma? To complicate things, the d2 value changes with the sample size, which means that sigma will also change if the sample size changes.

A process capability study is just a statistical guess of the chances that a variable might exceed its specification. It’s a calculated guess, not an absolute.

You can only calculate the true sigma when you have measured the entire population. Of course, then the work is done-the game is over-and what you have is what you have. So, that sigma is only usable as history or for analysis. Instead of calling it Six Sigma, you could and just make up a word and say it means 3.4 DPMO. “Depmo” sounds good. Another thing. Every statistical text book I’ve ever read, uses the word about when discussing sigma-the distribution of individuals under a normal curve. Unlike the table in the Six Sigma book, they say about 68% of the individuals will be within +/- one sigma away from the average. Not exactly 68%, but about 68%. About 99.7% will be within +/- three sigma and so on. In theory if you got out to +&- six sigma, you might get a number of about 3.4, or you could get more, or you might get nothing.

The Six Sigma method may be an effective tool for solving process problems and getting corrective action, that’s wonderful. But why the sigma nonsense? Quality Engineers have been doing that since the year one. Anyway, if you can reach 3.4 DPMO, why not just go ahead to Zero DPMO, or Zero Defects?

To read part II of Dave Crosby's blog, click here.
You must login or register in order to post a comment.

About the name

Ronnie van 't Westeinde
November 23, 2009
"Six sigma" alliterates really well, and is a catchy name. Mix in a blend of statistics, and it sounds amazing and stimulates the imagination. I think they made an excellent choice in naming it six sigma. (Although it's actually 4.5 sigma, but that doesn't sound too well)

About the name

Ronnie van 't Westeinde
November 23, 2009
"Six sigma" alliterates really well, and is a catchy name. Mix in a blend of statistics, and it sounds amazing and stimulates the imagination. I think they made an excellent choice in naming it six sigma. (Although it's actually 4.5 sigma, but that doesn't sound too well)

Problems with Six Sigma

Diane Kulisek
December 2, 2009
Interesting, Dave. I'm looking forward to reading about the next five problems you've got with Six Sigma. I do have one question for you, though. Are you sure you only have six problems with six sigma, or only six you're willing to talk about here? 666 problems with Six Sigma seems a more likely count. :::smiles:::

Small problem

Jim Taylor
December 3, 2009
To my thinking, the sigma calculation is a rather insignificant part of "Six Sigma", especially if Six Sigma is an effective tool for problem solving and getting corrective action. Don't throw the baby out with the bath water.

Comments on comments

Dave Crosby
December 3, 2009
. I hope you enjoy the next five and will comments to let me know where I'm going wrong. The reason I have six is because I thought it made a nifty title;I can come up with more. 2. I see the misuse of sigma as a reason to reject the whole concept as a hoax. In section six, I think I explain why it works at all. Thanks to both of you for the comments. Keep them coming.

Six Sigma

CLSOO
December 14, 2009
Very interesting article. I look forward to read more about the other 5 problems that you have defined with six sigma . The biggets challenge will be how do we use these Quality tools effectively; not doing it just for the sake of the customer's command .

Six Sigma

SH See
December 20, 2009
Before Six-Sigma, it was statistical method. It was then coined by some smart guy in Motorola to Six-Sigma. Technically, I still think "statistical method" is more accurate term to describe it. If you think marketing wise, something like Six-Sigma may sound logical.

It's all been said before

Six Sigma Belt Person
January 12, 2010
Before you go on and list all the rest of your grievances with Six Sigma, have a look at the Six Sigma Sucks blog entry by Andrew Downard: http://blogs.isixsigma.com/archive/six_sigma_sucks.html With 91 comments and counting, you'll read that all your problems with the methodology have been said before, and answered to already. So go on and continue to knock the Six Sigma methodology in favor of your own book, "the zero defects option." Because that's what you're selling right?

It's all been said before

Six Sigma Belt Person
January 12, 2010
Before you go on and list all the rest of your grievances with Six Sigma, have a look at the Six Sigma Sucks blog entry by Andrew Downard: http://blogs.isixsigma.com/archive/six_sigma_sucks.html With 91 comments and counting, you'll read that all your problems with the methodology have been said before, and answered to already. So go on and continue to knock the Six Sigma methodology in favor of your own book, "the zero defects option." Because that's what you're selling right?

What I'm selling

Dave Crosby
January 17, 2010
Yes, I'm selling my book, The Zero Defects Option. Why not? I believe in what I do. I've been trying to help people produce a quality product or service for over fifty years. When it's necessary, I have even given consulting time for free. I do that from my readers via email. The quality profession is full of good people, trying their best to protect their company. Programs like six sigma take the responsibility for quality away from the boss and give it to the quality or six sigma guy.

Sigma

Kevin Black Belt
January 20, 2010
The method is Define, Measure, Analyze, Improve, Control.. I have used the methods to solve multiple problems where "6 Sigma" was not the goal, identifying and eliminating variance thus saving money, time, and reducing waste was the goal.

QUALITY PROCESS ENHANCEMENT

ANITA
April 24, 2010
DETERMINE THE QUALITY AND ANALYSE THE PROBLEM NOT CONSIDERING ONLY VARIANCE BUT YOUR OWN QUALITY POINT OF VIEW AND INCULCATE IT WITH SOMETHING OUT OF THE BOX, INNOVATIVE AND REVOLUTIONARY.........

Multimedia

Videos

Podcasts

 In honor of World Quality Month, we spoke to James Rooney, ASQ Past Chairman of the Board of Directors 2013, for his take on quality around the world.
For more information, read the ASQ Speaking of Quality column.
More Podcasts

THE MAGAZINE

Quality Magazine

cover_image

2014 April

Check out the April 2014 edition of Quality Magazine for features!
Table Of Contents Subscribe

Manufacturing Process

Has/does news about a manufacturers’ recall (like the GM recall in the news now) cause you or your company to reexamine its manufacturing process?
View Results Poll Archive

Clear Seas Research

qcast_ClearSeas_logo.gifWith access to over one million professionals and more than 60 industry-specific publications,Clear Seas Research offers relevant insights from those who know your industry best. Let us customize a market research solution that exceeds your marketing goals.

eNewsletters

STAY CONNECTED

facebook_40.png twitter_40px.png  youtube_40px.pnglinkedin_40px.png