Six Sigma is a fact-based, data-driven philosophy of quality improvement that values defect prevention over defect detection. It drives customer satisfaction and bottom-line results by reducing variation and waste, thereby promoting a competitive advantage. It applies anywhere variation and waste exist, and every employee should be involved.

In simple terms, Six Sigma quality performance means no more than 3.4 defects per million opportunities.

Several different definitions have been proposed for Six Sigma, but they all share some common themes:

• 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 (PDF, 755KB). Review the different Six Sigma belts, levels and roles.
• Emphasis on the DMAIC approach (define, measure, analyze, improve and control) to problem solving.
• A management environment that supports these initiatives as a business strategy.

Differing opinions on the definition of Six Sigma:

Six Sigma is a philosophy— This 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).

Six Sigma is a 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 flowcharting.

Six Sigma is a 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.

Excerpted from Donald W. Benbow and T. M. Kubiak, The Certified Six Sigma Black Belt Handbook, ASQ Quality Press, 2005, pages 1-2.