To do my part to continue to educate people on the differences between lean and 6-sigma, I have listed some of the areas where the two approaches contradict one another.
Transformation: Lean is a systems approach to business that runs so counter to traditional thinking that it virtually always requires some level of transformation. In fact, attempting to deploy lean without transformation will result in a lot of effort with little or no improvement. 6-sigma, on the other is based on individual improvement projects and does not require major shifts in thinking or changes to the company's culture, systems, and approach to leadership.
Source of Improvement: 6-sigma uses "experts" (or black belts) to lead projects that attack problems. In some cases, leaders do not need to be involved in addressing problems in their areas. Lean gets everybody involved in addressing problems. The leader of an area is the expert who is tasked with coaching and developing team members in the way to identify and attack problems. Someone not involved in the area affected by the problem may be called in as a coach, but won't be directly involved in the effort. [See Solutions below for more on the idea of solving problems ]
Size of Problems: 6-sigma uses a set of criteria to focus attention on the big problems. Pareto charts are commonly used to separate the critical problems that require attention. Lean attacks any problem that interferes with meeting a target condition, regardless of size. Although Pareto is used occasionally in a lean environment, people do not wait for data to be collected to prove an issue is big enough to warrant attention and are encouraged to attack problems as they occur.
Solutions: 6-sigma uses the DMAIC process (Define-Measure-Analyze-Improve-Control) to "solve" problems, while lean applies PDSA cycle (Plan-Do-Study-Act) cycle to develop countermeasures with the idea that, under a continuous improvement mindset, no problem is ever solved. Some consider this as merely semantics, but it is an important part of establishing a lean mindset. Thinking of a problem as "solved" can lead to complacency and a level of satisfaction that can kill the drive to further improve.
Learning: 6-sigma relies heavily on classroom training and certification to drive learning. Lean involves a "just do it" attitude and places more emphasis on coaching than training. Also, within a lean environment, the continual developing and testing of hypotheses, rather than classroom training, drives learning. An important responsibility of leadership within a lean environment is to develop those on his or her team to think and practice PDSA thinking, where 6-sigma tends to use trainers who are outside of the operation to develop problem-solving skills.
Since lean is transformational - requiring significant shifts in systems, policies, learning, and leadership - it unlocks what W. Edwards Deming referred to as the big gains. Deming surmised that using the tools without transformation can result in no more than 3% improvement for a company. The other 97% is locked in the company's systems, including leadership, planning, strategy, etc.
I believe that the key difference between the two practices is that 6-sigma attacks the 3%, while lean aims at the 97% (along with the 3%). It naturally follows, however, lean is much more difficult for an organization to understand and practice. Lean also aims at many of the traditional western business practices of which leaders can be reluctant to let go.
Whether embarking on the deployment of 6-sigma or lean, it is important to be clear on expectations. If you're not ready to question the organization's systems and leadership, stay with 6-sigma and be happy with the 3% - which admittedly, can be quite large in financial terms. If you want the big gains, though, lean - not 6-sigma - is the way to get there. In other words, don't expect 97% results by using a 3% solution.