The Interaction of Lean Thinking, Six Sigma, and Theory of Constraints

Over the years there have been three major continuous improvement methodologies developed independently in order to address the issues of improving processes.

1. The Toyota Production System (TPS) or what came to be known as Lean Thinking as portrayed by Womack and Jones in 1995 started to be developed by Taiichi Ohno in the late 1940s.
2. Theory of Constraints (TOC) was developed by Eli Goldratt and burst upon the world in 1984 as The Goal for manufacturing processes. Mr. Goldratt very effectively applied this same mindset in subsequent work for the project management environment and many other common business applications.
3. And finally, in the 1990’s, Six Sigma was developed by Motorola in an effort to compete with the Japanese electronics companies who had embraced elements of these earlier methodologies.

In the last few years, practitioners have come to understand that these are not, in fact, independent methodologies. A way to characterize the interrelationship of these methodologies is to consider the story of the seven blind men describing an elephant: the one that has a hold of the elephant’s tusk declares that the elephant is “like a spear”; the one that has a hold of the elephant’s foot declares that the elephant is “like a tree.”; the one that has a hold of the elephant’s tail declares that the elephant is “like a rope”; and so on. They are all correct; however each has a different perspective on the beast! Similarly, the improvement methodologies are all looking at the same “beast”: Continuous Process Improvement!

In fact, these methodologies are complementary: they each have strengths that complement the other’s weaknesses… or omissions. Practitioners of each of these methodologies are frequently unaware that there are weaknesses or omissions because each of the methodologies on its own works very well. It is just that the three of them work better together!

Whereas, both TOC and Lean Thinking are more qualitative, Six Sigma is quantitative beyond belief (though not to a Six Sigma practitioner in isolation). Improving a process typically starts with a “bottleneck” analysis for an entire system from the TOC perspective and proceeds to the Lean Five Steps.

There is so much “low hanging fruit” when you go into a new organization that has not been applying any of these methodologies, that you typically start out with TOC and Lean. Applying these tools sets together makes the processes throughout the organization more stable, more reliable, much better documented and much simpler. THEN, as the organization’s core processes stabilize, you progress into the Six Sigma methodologies for critical processes that TOC and Lean just don’t have the tools to address. For example, both TOC and Lean are much better at identifying and elevating the one central constraint (or at most, two central constraints) of a process, whereas Six Sigma excels at quantitative analysis and sustaining the improvement.

You in fact need an option of tools to accomplish all of these functions. Trying to apply Six Sigma without a very clear understanding of the central constraints in a system is hurting the Six Sigma movement. Yes, it is extremely good at improving specific elements of a system, however, as we learned from the Theory of Constraints, improving ANYTHING that is not the “bottleneck” or central constraint of the system is a mirage… it is fake, it is misleading. It looks like improvement, but it does not produce more of what the process was designed to do because you still have the original process constraint as well as the original performance measurements. Evaluating changes on a process without careful evaluation of the performance measures for that system, especially the financial and accounting measures, is just asking for trouble.

Referencing Reza Pirasteh, Ph.D. in the May 2006 issue of APICS Magazine, the essential Theory of Constraints, Lean, 6 Sigma methodology (69k pdf) is to apply TOC tools to identify the system constraints, and then Lean and Six Sigma tools to eliminate those constraints so that they are no longer systemic constraints, and then do it all again by applying the TOC tools to identify the next system constraint.

The TTZ, on this site, is a flow chart, along the Value Stream Mapping lines of Lean Thinking, that gets the TOC focus applied to it over and over again, no matter where the constraint lies in the entire organization… followed by Lean and Six Sigma tools, preferably in rapid sequence… like a tornado! (Please see the Helix article.)