What is Six Sigma?

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Quality Glossary Definition: Six Sigma

A method that provides organizations tools to improve the capability of their business processes. This increase in performance and decrease in process variation lead to defect reduction and improvement in profits, employee morale, and quality of products or services. Six Sigma quality is a term generally used to indicate a process is well controlled (within process limits ±3s from the center line in a control chart, and requirements/tolerance limits ±6s from the center line).

Free Six Sigma Resources

Delivering Process Excellence Through Process Management (webcast)

Costs and Savings of Six Sigma Programs: An Empirical Study (article)

Molding a Solution (case study)


Different definitions have been proposed for Six Sigma, but they all share some common threads:

  • Use of teams that are assigned well-defined projects that have direct impact on the organization's bottom line.
  • Training in "statistical thinking" at all levels and providing key people with extensive training in advanced statistics and project management. These key people are designated “Black Belts.” Review the different Six Sigma belts, levels and roles.
  • Emphasis on the DMAIC approach to problem solving: define, measure, analyze, improve, and control.
  • A management environment that supports these initiatives as a business strategy.

Differing opinions on the definition of Six Sigma:

Philosophy— The philosophical perspective views all work as processes that can be defined, measured, analyzed, improved and controlled. Processes require inputs (x) and produce outputs (y). If you control the inputs, you will control the outputs. This is generally expressed as y = f(x).

Set of tools— The Six Sigma expert uses qualitative and quantitative techniques to drive process improvement. A few such tools include statistical process control (SPC), control charts, failure mode and effects analysis, and process mapping. Six Sigma professionals do not totally agree as to exactly which tools constitute the set.

Methodology— This view of Six Sigma recognizes the underlying and rigorous approach known as DMAIC (define, measure, analyze, improve and control). DMAIC defines the steps a Six Sigma practitioner is expected to follow, starting with identifying the problem and ending with the implementation of long-lasting solutions. While DMAIC is not the only Six Sigma methodology in use, it is certainly the most widely adopted and recognized.

Metrics – In simple terms, Six Sigma quality performance means 3.4 defects per million opportunities (accounting for a 1.5-sigma shift in the mean).

Excerpted from T. M. Kubiak and Donald W. Benbow, The Certified Six Sigma Black Belt Handbook, 2nd edition, ASQ Quality Press, 2009, pages 6-7.


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