A Tale of Two Theories – LEAN or Six Sigma

A Tale of Two Theories

Have you heard the term ‘Lean Six Sigma’?  Of course you have. But wait!  Did you know that ‘Lean’ and ‘Six Sigma’ are actually two different methodologies?  Alright, you probably had some idea of that too. Why, then, is it that we so often intertwine these two cryptic workplace improvement phrases with little regard for their individual identities?  Well, that’s exactly what we’re going to be discussing today.

Linkedin user David Kenny, in a nod to the small distinctions between the two ideologies, recently asked users to pick and defend a side. In short, he asked participants if they would deem Lean or Six Sigma more important if they could only choose one to use in their businesses. Another discussion point was whether one of the two was able to be labeled as more of a “subset” or “tool” of the other. While there were a couple of outliers, the general consensus was a resounding, “ummm, I don’t know, they seem pretty similar…”

And you know what?  Such a reaction is understandable; not only are the goals of these two methodologies very closely aligned, the way that they’re used in the common workplace/business vernacular isn’t always indicative of their strictest roots and definitions.


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First and foremost, Lean concerns itself with waste elimination. This has grown to mean any number of things, but Toyota, founders and pioneers of Lean, defined seven distinct categories of waste: Waiting, over-production, over-processing, defects, inventory, waiting, motion, and transport.

Lean methodology asserts that by identifying the sources of each of these kinds of waste and applying appropriate solutions, one can move the production cycle toward continuous improvement, one of the three primary goals of Lean (along with challenging oneself to meet set goals and basing those goals in sound fact by visiting the “factory floor” – gemba walks, etc.).

Six Sigma

Six Sigma, wouldn’t you know it, also wants to make your business run smoother!  The big difference between the two is in how they identify the cause of slow-downs. Whereas Lean focuses on various pieces of waste, Six Sigma seeks to be the catalyst for improvement through eliminating variability. Variability can lead to high rates of defection, uncertainty which can cause unnecessary waiting, etc.

Common Usage

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Wait a minute, those last few symptoms of “variability” sure look close to the goals for elimination (types of waste) we identified in the Lean paragraphs, don’t they?  This was a common crux of the argument on LinkedIn which sought to pair the two together in a parent/child relationship; it could be said that much of the waste to be eliminated via Lean practices was really symptomatic of variability, i.e. a root cause. On the other hand, some argued that Six Sigma was more of a tool of Lean for identifying problems in the sub-category of variability.

Driving this confusion is the fact that the terms are used pretty much interchangeably on a daily basis, even from those who know there’s a distinction. One possible reason for this is the fact that they are seen as working toward the same goal, and therefore both just cogs in the continual improvement machine. Here are a few statements from the discussion’s posters illustrating as much:

“I have to agree with Jeffrey. This isn’t a choice that has to be made. I do not agree with those who say they are rival methodologies.” – Tim Rodgers

“I think it depends upon how one structures it in the form of methodology. All things are intertwined.” – Ravi Sankar Duvvuri

“Together they are a means to accomplishing reduction in cycle time, increase in quality, and reducing overall cost without making any compromise to any to achieve all three.” – Joseph Campagna

Further complicating the issue is that tools that were once thought of as specifically associated with either Lean OR Six Sigma are now being used for both. One example is the DMAIC framework, which works through Define, Measure, Analyze, Improve, and Control. This method originated alongside Six Sigma, but is now commonly suggested as a possible mapping tool for Lean operations. Heck, even the fact that the combined “Lean Six Sigma” hybrid name exists today is telling as to the working relationship the two have.

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The reality of the situation is that the distinctions made between the two are arguably less and less relevant as time goes on; both powerful ideas complement each other, and whether one is “more important” than the other or not is ultimately a non-factor as long as the desired outcomes are accomplished.

That said, it is important to, no matter the name they go by, keep the most important principles of each in mind so that they continue to be used. For example, we’d hate to see any focus on variability phased out if it comes to simply be viewed as another form of waste rather than an equally important and directly correlating factor.

In the real world, practitioner usage features a balance of the two approaches in most cases. While looking for ways in which time and resources are being wasted throughout an operation, the variability will come up whether or not it is explicitly described as such.

If you notice that some days have more waste than others, that’s variability. If you notice that some employees have faster or more efficient work routes than others, that’s variability. If a certain machine is producing more defective or unusable parts than others, you guess it, variability again.

When put in a more illustrative context like this, it’s easy to see how both Lean and Six Sigma are integrated into the problem identifying process.

When it comes to actually implementing solutions, this is a phase where processes and definitions are bent, as your quest to reduce waste and variability will inevitably find you using tools that may be traditionally more associated with one or the other. In the end, this is, at least in my opinion, insignificant, and has little bearing on the good work you’re doing, nor the effectiveness of your results.

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