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What is the difference between Lean and Six Sigma?

What is the difference between Lean and Six Sigma?

Hang around the Lean community for any length of time, and you will start to hear about the rivalry between Lean and Six Sigma. The two methodologies are the juggernauts of the continuous improvement world. While they are both focused on making operations work more smoothly, they have a slightly different approach in how they do that.

The traditional view is that Lean is focused on pulling wasteful steps and inventory out of a process, and Six Sigma is focused on removing variation. While that is true to some degree, neither methodology limits itself to just one or the other.

The truth is that there is a tremendous amount of overlap in the tools and processes of the two philosophies of improvement. For example the DMAIC cycle in Six Sigma closely parallels the PDCA cycle and its derivatives like the A3 report. A further similarity is that rather than being just toolkits, Lean and Six Sigma are both structured ways of thinking and problem solving.

As for the primary emphases of the methodologies, by no means should they be considered boundaries. Lean must reduce variation to make Standard Work effective and for kanban systems to run smoothly. When six Sigma projects pull variation of a process, it tends to take a great deal of waste with it.

As for differences, the biggest ones between the two lie in scope and in accessibility. Six Sigma contains a set of sophisticated statistical tools that are generally too complicated for most front-line employees to use effectively. Lean, on the other hand, uses tools that are fairly easy for anyone to grasp. For that reason, there are generally fewer Six Sigma experts in a company that people who can take on Lean projects. As a result, Lean projects can be of virtually any size and are more widespread.

Another reason that Six Sigma tends to be used for bigger projects is greater degree of certainty that its tools provide. This helps mitigate the risk of taking action when the stakes are high. Lean tends to promote taking action with a somewhat lower threshold of confidence. That’s not to say that Lean is done by the seat-of-the-pants. It just means that ANOVA is likely to be more accurate than a Pareto chart or a run chart, and can unearth more subtle nuances of a problem.

Because of the overhead on data collection and the limited number of people who can crunch the numbers, Six Sigma tends to be reserved for bigger projects with greater potential impact. Lean tends to be more commonly implemented with daily improvement and weeklong kaizen events.

The bottom line is that there are more similarities than differences in the approaches used by the two methodologies. In fact both have their own sets of strengths and weaknesses. For that reason, in recent years, a blend of Lean and Six Sigma has been taking root. Not surprisingly, this hybrid is known as either Lean Sigma or Lean Six Sigma. 


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