Lean Manufacturing and Six Sigma Definitions

Glossary terms, history, people and definitions about Lean and Six Sigma

Six Sigma

Six Sigma is a disciplined, statistical-based, data-driven approach and continuous improvement methodology for eliminating defects in a product, process or service. It was developed by Motorola and Bill Smith in the early 1980’s, along with help from Mikel Harry and Mario Perez-Wilson (view Six Sigma timeline from 1984-1994). It is based on quality management fundamentals, which became a popular management approach at General Electric (GE) with Jack Welch in the early 1990’s. The approach was based on the methods taught by W. Edwards Deming, Walter Shewhart and Ronald Fisher among many others. Hundreds of companies around the world have adopted Six Sigma as a way of doing business.

Sigma represents the population standard deviation, which is a measure of the variation in a data set collected about the process. If a defect is defined by specification limits separating good from bad outcomes of a process, then a six sigma process has a process mean (average) that is six standard deviations from the nearest specification limit. This provides enough buffer between the process natural variation and the specification limits.

For example, if a product must have a thickness between 10.32 and 10.38 inches to meet customer requirements, then the process mean should be around 10.35, with a standard deviation less than 0.005 (10.38 would be 6 standard deviations away from 10.35), assuming a normal distribution. See the example below.

Six Sigma can also be thought of as a measure of process performance, with Six Sigma being the goal, based on the defects per million. Once the current performance of the process is measured, the goal is to continually improve the sigma level striving towards 6 sigma. Even if the improvements do not reach 6 sigma, the improvements made from 3 sigma to 4 sigma to 5 sigma will still reduce costs and increase customer satisfaction.


Here are some examples of processes from different industries and their actual sigma levels.

The video below provides a simple explanation of the different sigma levels, and how they relate to golfing.

The martial arts belt structure is used to recognize proficiency in training and application in Six Sigma, using the following colors:

  • White Belt – Overview, DMAIC, Define Phase
  • Yellow Belt – White Belt + process mapping, data collection and charting, assisting with a project
  • Green Belt – Yellow Belt + Project leader, core Six Sigma tools (Gage R&R, SPC, Capability, ANOVA, Regression), change management, hypothesis tests and more
  • Black Belt – Green Belt + advanced statistical analysis and experiments, change management, nonnormal distributions
  • Master Black Belt – Black Belt + Design for Six Sigma, more advanced statistical analysis, unique tools for specific industries and processes, working with leadership, implementing successful improvement programs

However, not all levels are consistent and equal to each other, so it is important to ask questions about the topics covered and requirements needed to complete each belt level.

Formal certification is recognized at the Green Belt, Black Belt and Master Black Belt level based on one or more of the following criteria:

  • Completion of training covering the body of knowledge
  • Years of work experience with the body of knowledge
  • Passing an exam covering the body of knowledge
  • Completion of one or more Six Sigma projects

Certification is typically authorized by consulting firms or industry organizations and membership groups.

A combined Lean Six Sigma belt typically is an expanded or modified version of Six Sigma training with the addition of Lean principles and tools.

Want some ideas for Six Sigma projects you can run within your industry? Check out this list of Six Sigma project ideas to help get you started…

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Additional Resources