What Does Six Sigma Mean?

So, Just What Does “Lean Six Sigma” Mean?

For the past 30+ years, the Lean Six Sigma (L6S) improvement process has been used to boost effectiveness and make a positive impact on the bottom line for both manufacturing and service companies. While it’s generally assumed that applying these tested methods should lead to profit increases between 10% and 40%, the results in individual organizations don’t always live up to expectations.  One reason for such subpar performance may be due to a misunderstanding of the definitions for L6S.

Since Motorola coined the phrase “Six Sigma” over 20 years ago, it has become part of our common language and has acquired many definitions over the years.

Most continual improvement practitioners, Lean Six Sigma Belts, and other quality professionals would agree that there are three fairly common definitions of the phrase “Lean Six Sigma.” These definitions are the subject of this article.

Definition 1: A Philosophy of Continual Improvement

Companies that claim they are a “Lean Six Sigma” company don’t generally intend to imply that all their processes are performing at a level of 3.4 defects per million opportunities.

What they mean is that over the years they have integrated a philosophy of continual improvement into the DNA of their company.

Not only do they have multiple ongoing Lean Six Sigma projects, but the practice of continual improvement is present in everything they do. Every process they perform, every product / service they produce, and every client they serve has been affected by continual improvement.

The tools used in the Lean Six Sigma processes of DMAIC and DMADV have been integrated into every aspect of their work. Their decisions are data driven and the customer is king.

These are true “Lean Six Sigma” companies. 

Definition 2: A Measure of Variation

The phrase, “Lean Six Sigma” sometimes is written as “6σ.” This contains the lower case Greek letter, sigma. For those of you not familiar with it, this is the symbol for the standard deviation of a population.

Sometimes, when we’re talking about “Lean Six Sigma,” we are simply discussing variation. It may be the natural variation in the process, the variation in the product, or the variation in meeting customer requirements.

We would normally use “s” to represent this variation since most data is a sample of the population, but the use of “σ” has made its way into the lexicon.

There is a difference in the way “s” and “σ” are calculated and it’s important to use the correct equation.

With Definition 2, generally speaking, the smaller the value, the better the process.

Definition 3: A Process Performance Metric

This is the most commonly used definition of “Lean Six Sigma” and the typical reference is the 3.4 defects per million opportunities. It is a true Quality metric, like capability indices, since it describes “how well” the process is performing in relation to the requirements.

The metric can be used regardless of process (manufacturing and service) or data type (variable and attribute).

The calculations use the critical customer requirement (CCR) and process performance statistics.

For attributes data we use the Defects Per Million Opportunities (DPMO) calculation. The numbers needed from the process to calculate DPMO are the number of defects, the number of defect opportunities, and the number of units.

The equation is:

                                                Number of Defects

         DPMO  =           ----------------------------------------------------     x  106

                                    Defect Opportunities x Number of Units


Once DPMO is calculated we can reference a conversion table to find the corresponding sigma value.

For variables data, the average and standard deviation of the process are the statistics needed. We use the standard z calculations for the corresponding sigma value. The calculations are slightly different for a one-sided specification than for a two-sided specification.

In both the variables and attributes data calculations, we end up with a sigma value.

Using Definition 3, the larger the value, the better the process. A “4σ” process is better than a “2.5σ” process. This is opposite to what we desire in Definition 2. In that case we want the sigma value to be as small as possible.

To reduce confusion in any discussion, it is critical that we have a common understanding of which definition of “Lean Six Sigma” is being referenced.

It should be noted that there are quite a few quality professionals that disagree with this calculation for a variety of reasons.

I think it’s another useful tool to look at process performance.

In summary, there are three common definitions for the phrase “Lean Six Sigma”:

  1. A Philosophy of Continual Improvement
  2. A Measure of Variation
  3. A Process Performance Metric

In order to have meaningful discussions, it is important to understand which definition is being used.

So, how does understanding the definitions impact the bottom line for your organization?  By starting with the right baseline, and implementing the right principles, you can expect to see results mentioned at the beginning of this article. 

For this reason, I always recommend seeking the input of an experience LEAN Six Sigma consultant when shaping your strategy, and training your employees at all levels of the organization to understand and apply that strategy according to best practices.