What is Six Sigma?

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Quality Glossary Definition: Six Sigma

Six Sigma is 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).


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).

What is lean Six Sigma?

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

The demarcation between Six Sigma and lean has blurred. We are hearing about terms such as “lean Six Sigma” with greater frequency because process improvement requires aspects of both approaches to attain positive results.

Six Sigma focuses on reducing process variation and enhancing process control, whereas lean drives out waste (non-value-added) and promotes work standardization and flow. Six Sigma practitioners should be well versed in both.

Integrating lean and Six Sigma

Lean and Six Sigma have the same general purpose of providing the customer with the best possible quality, cost, delivery, and a newer attribute, nimbleness. There is a great deal of overlap, and disciples of both disagree as to which techniques belong where.

The two initiatives approach their common purpose from slightly different angles:

• Lean focuses on waste reduction, whereas Six Sigma emphasizes variation reduction

• Lean achieves its goals by using less technical tools such as kaizen, workplace organization, and visual controls, whereas Six Sigma tends to use statistical data analysis, design of experiments, and hypothesis tests

For ASQ members: Webcast series on lean Six Sigma Tools

The most successful users of implementations have begun with the lean approach, making the workplace as efficient and effective as possible, reducing waste, and using value stream maps to improve understanding and throughput.

When process problems remain, the more technical Six Sigma statistical tools may be applied. One thing they have in common is that both require strong management support to make them the standard way of doing business.

Some organizations have responded to this dichotomy of approaches by forming a lean-Six Sigma problem-solving team with specialists in the various aspects of each discipline but with each member cognizant of others’ fields. Task forces from this team are formed and reshaped depending on the problem at hand.

Learn more about combining lean and Six Sigma

Lean and Six Sigma—A One-Two Punch (PDF, open access)

Using the Six Sigma/kaizen team-based approach, results are implemented faster with the participation of employees from the shop floor to the executive suite.
Better Together (PDF, open access) A simple group demonstration allows participants to gain an understanding of the advantages offered by lean and Six Sigma when they work together
  See more resources on lean and Six Sigma.

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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