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what is sigma

Oct. 15, 2011

Permalink 12:28:00 am, by Michael, 815 words   English (US) latin1
Categories: Project Digest, On Six Sigma

What is Sigma

There's no better place to start our Six Sigma project than to explain a bit about "what is sigma" and then more specifically what is Six Sigma.  Did you know that the global search volume for the key word "Six Sigma" is 1,000,000/month!  That’s one million people searching the term Six Sigma every month.  A fair volume of interested folks but that’s not the really interesting part, the really interesting part is that there are 11,000,000 searches for the term "what is sigma" and another million who search for the term "what is six sigma".

It's safe to say that millions of people wonder what six sigma is, maybe it's because very few people can explain it simply and do it justice at the same time.  Honestly, I struggle with it myself but I'll give it a whirl despite the fact that it's like trying to nail a cube of Jell-O to a tree!

Six Sigma is simply a goal, to perform an operation successfully 99.99967% of the time.

Imagine ONE MILLION attempts and

  • missing only 3.4 putts
  • missing only 3.4 foul shots
  • missing only 3.4 field goals

That would be exceptional...and in fact, that would be Six Sigma.  Yes, that's the craze! and companies aspire to achieve ever increasing levels of production efficiency and quality every day.  They use automation, process standards, secondary and tertiary systems and other variation reduction techniques that create instances all over the globe where Six Sigma is already achieved.  To add some perspective, it's a lofty goal but it's also achievable in many industries.

The airline industry is one such industry.  Over the past 25 years the commercial airline industry has operated at 6.61 Sigma in terms of fatal incidents.  6.61 Sigma is a fatal incident rate of 0.0000163% or 0.163 incidents per million flights.  But what if the industry only operated at a 99% success rate (about 3.83 sigma)?  That would mean over 5,200 fatal incidents per month!  Yes, you're reading that correctly, there were an average 523,466 commercial domestic flights per month for the past 25 years and at a 99% success rate, 1% or 5,235 of them would result in a fatal incident every month.    But that's not our reality.  When lives are on the line, we ensure safety measures yield adequate risk tolerances by using error proofing, automation, secondary systems and in some cases tertiary systems.  These expensive countermeasures result in a transportation method that is one of the safest available.  In turn, we have an industry that demonstrates that Six Sigma is achievable.

Now you’re probably wondering what about the "sigma" part and why six?   Well, sigma is just a term used by statisticians to refer to variation.  You'll hear this often as you follow along with me, variation is the enemy.  Take for example your favorite pair of jeans.  They are probably your favorite mostly because at some point they fit so perfectly that you fell in love with them.  But, how many times did you try on the same size jeans in the same brand and they all fit differently?  That's the variation in production processes that Six Sigma intends to eliminate.  How nice would it be to know that you could buy the same brand of jeans in the same size and get the same fit every time?  Yeah i know!

Most processes have specification limits or targets to meet.  Your jeans for example might have a waist circumference labeled 34 inches but the production process that cut them might have specification limits to stay within e.g. 33.85 - 34.15 inches.  Sigma refers to the number of standard deviations between the process average and those specification limits.  If the production process can be precise enough to cut the jeans on average at 34.01 inches with a standard deviation of 0.04 inches then it is likely that they would stay within the specification limits 99.99967% of the time or Six Sigma.  The image below depicts a process operating at 6 sigma.  Notice there is plenty of room for the process to shift around still not product defects.

On the other hand, the cut can average 34.00 on the nose with a higher or wider standard deviation which means that the cutting process may render cuts outside of the predefined specification limits ("out of spec").  Out of spec conditions are defects and we want to avoid them.  See the image below depicting a process operating at approximately 4.25 sigma.  The red "tails" of the distribution are outside of the specification limits indicating defective products.

So there you have it, the longest answer to a simple question you never wanted to hear!  and this is just the beginning :-).  Lets back up and rewind a little before we move on, please remember this...

Six Sigma is just the goal.

There is something else very important that you're going to learn on the heels of this simple but important concept.  It's that there is a proven and well-structured method you can use to achieve Six Sigma.  The method is D.M.A.I.C and we'll learn that next.

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