Six sigma vs Lean. Whats the difference?

As you consider your options for kicking off a Continuous Improvement initiative, one of the questions you'll face is which program to chose. I talked a little about this here. Today, I want to go a little more in depth on the differences between two of the most popular CI programs; Lean and Six Sigma.

First a little definition, starting with Six Sigma. Six Sigma is many things. It is a statistic (3.4 defects per million), it is a set of tools (GR&R, Descriptive Stats, Regression, DOE, etc.), it is a problem solving process (DMAIC, PIDOV, DMADV), but it is also a management philosophy.

Lean is all of the things listed above (except a statistic) for Six Sigma. Lean is a toolset (Kanban, Poke Yoke, Andon, VSM, Standard Work, Visual Factory, etc..).Lean is also a process, usually starting with 5S and proceeding from there to more difficult problems and solving them using a Kaizen approach. Lean is also a management philosophy.

Both approaches can be successfully applied at any of the levels mentioned, but the full realization of each is as a management philosophy. Gains can be made when the tools are applied to a given problem so each can be successful without full commitment, further each compliments weaknesses in the other approach, which leads to the recent trend in combining the tools and methods together (Lean Six Sigma). There are obvious differences between the tools used in each method, the processes used are similar to each other. Kaizen can be mapped over the steps of DMAIC or vice versa.

So what is the difference between Lean and Six Sigma? The biggest differences can be found when Lean or Six Sigma is applied pervasively as a way of managing the business. Both include training as a core component but there is a subtle difference between the approaches used. While both Lean and Six Sigma need trained practitioners to be successful, Lean has an additional value placed on training that does not exist in Six Sigma. A core concept of a Lean culture is a cross-trained workforce. Cross-trained in the work process, not specifically in Lean tools. This is an important part of a Lean Transformation because a cross-trained workforce supports job rotation to keep workers from getting bored and complacent. Another significant difference is evident in the use of measurements. Both Lean and Six Sigma emphasize measuring performance, the approach is very different however. Six Sigma seeks to measure process performance relative to the problem being worked on at the project level or of the most important measures of business success through Dashboards. These dashboards cascade down from the top and rarely show up on the shop floor, so the workers dont really know how they are doing. Lean, on the other hand, focuses measurements on the shop floor FIRST and places a high value on managers going to the Gemba, or shop floor to see for themselves how things are going. This suggests a bottoms up approach to measurement, starting with the work cell and proceeding UP to the plant and the company from there. This is supported by Visual Factory concepts. I remember a quote from one of my Lean Sensei's that the standard for Visual Factory effectiveness was that a one eyed person could gallop a horse through the plant and at the end be able to tell you what is going on in that plant. It should be that obvious. This difference highlights the main philisophical difference between Lean and Six Sigma. Six Sigma is a tops-down management philosophy that only occasionally reaches the shop floor whose main practitioners are mid-level managers and engineers. Six Sigma projects are chosen by reviewing performance at a high level and imposing the change down with some involvement from the "doers". Lean, on the other hand is characterized by the idea of "servant leadership" in which the leadership views one of its main functions to be enabling the success of the workers. Management goes to the floor to see what is happening, values effective communication, training, promotion, and learning and the initiative start and complete improvment work starts at the lowest level, the individual worker and the work cell member's desire to improve their own work.

Finally, there is one other significant differnce between Lean and Six Sigma at top level. Since Lean is so focused on enabling the worker to perform better, a sacred trust is built between the workers and management. This implied contract is that making Lean improvements will not result in people losing their jobs. This is incredibly powerful in creating trust. Dont get me wrong, Lean does result in fewer workers being needed in the work cell, but the sacred trust means that those workers are placed elsewhere, not on the street. Six Sigma on the other hand...well, one only needs to talk to someone who has worked for GE during the Welch era to know what happened there.