What Is Six Sigma?
How Does It Fit With Lean?

It is not easy to give a short answer to the question, what is Six Sigma. That is probably because Six Sigma can have multiple meanings depending on the context you use it in. Six Sigma can usually be categorized under two main definitions - the metric and the methodology.

Six Sigma - The Metric
The metric is a measurement of how good the quality of a process is. The higher the Sigma level, the better the process quality is. The metric version of Six Sigma is being used when somebody says, "My stamping process is a six sigma process." If a process produces less than 3.4 defects per million parts produced, it is considered to be a Six Sigma process. This means that the process is working with 99.99966% accuracy. This level of accuracy is thought to be the pinnacle of quality in many industries. Of course, it is possible to have a process working at better than Six Sigma level…but that is usually not required in most industries. Trying to get the quality levels higher than this may require a lot of time and money with very little return. Some industries, like the airline industry for example, are required to be and are currently at higher than Six Sigma level. After all, when human lives are concerned, even a 0.00034% chance that a plane may crash is too high.

So how does 3.4 defects per million parts equate to 6 sigma? This has to do with the normal distribution and the cumulative distribution function associated with it. Sounds fancy! But it's not that complicated. Click here to see how six sigma calculation is done.


Six Sigma - The Methodology
The methodology is probably what Six Sigma has become most well-known for today. It is a five phase methodology which uses a scientific and data-driven approach to find the optimal solution to a business problem. The five phases in order are Define, Measure, Analyze, Improve, and Control. Lovingly, we call it the DMAIC methodology. The DMAIC methodology is a very structured way of defining a business problem, finding out what needs to be fixed or changed, getting the answers on how to change it, implementing the changes, and then making sure the changes beneficial and sustainable. It is a very powerful methodology which many world class companies have embraced and have entrenched into their culture to be the most effective method for continuous improvement and problem solving. For more in-depth information on each of the phases and how they all come together, please see our guide to the steps in the steps in Six Sigma methodology.

The Six Sigma metric and methodology come together when you understand the objective of the Six Sigma initiative. To finally understand what is Six Sigma, you have to understand its objective. Traditionally, Six Sigma's focus has always been on reducing variation and improving process capability to reduce defects. To understand this better, please click here for The Objectives of Six Sigma tutorial.


Six Sigma and Lean
Six Sigma has gone through quite an evolution through its short history. When the concept was first implemented, it was purely a quality initiative. The question was always, "How do we reduce the number of defective products?" Over the years, the concept of quality itself evolved and was no longer just associated with defective products but quality became associated with the full customer expectation or specification. This now included things such as delivering to the customer on time, providing product at the expected price, providing the expected level of customer service, etc. This is when it really started to make perfect sense to combine Lean and Six Sigma. Lean, with its customer value focus, could provide the supplement to make Lean Six Sigma the most well rounded and effective way to manage today's definition of "quality".

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