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Six Sigma

Definition of Six Sigma

  • A Greek letter used to represent the distribution or spread about the mean (average) of any process or procedure
  • A measurement system. It provides a set of common measurements (benchmarks) that apply to all processes
  • A management philosophy

Critical to QualityCustomer Performance requirements
Characteristics (CTQ)of a Product or Service
DefectAny Event that does not meet the specifications of a CTQ
Defect OpportunityAny Event which can be measured that provides a Chance of not meeting a Customer CTQ

The term sigma is used to designate the distribution or spread about the mean (average) of any process or procedure.

For a business or manufacturing process, the sigma capability (z-value) is a metric that indicates how well that process is performing. The higher the sigma capability, the better. Sigma capability measures the capability of the process to perform defect-free work. A defect is anything that results in customer dissatisfaction.

Sigma LevelPPM
Process CapabilityDefects per million Opportunities

Six Sigma Philosophies

The Science: We don't know what we don't know.

The Art: If we can't measure it, we really don't know much about it. If we don't know much about it, we can't control it.

The Magic: If we can't control it we are at the mercy of chance

Statistical Objectives of Six Sigma

The statistical objectives of Six Sigma can be best understood from the diagram below:

Six Sigma Objectives

Advantages of implementing six sigma

The table below enumerates the improvement possibilities after six sigma implementation.

The classical view of Quality
99% Good (3.8σ)
The Six Sigma view of Quality
99.9996% Good (6σ)
30,000 ton of defective steel production in a steel plant every year.1200 ton of defective steel production in the steel plant per year
87 hours of Power supply failure in a machine shop per year3 hours of Power supply failure in the machine shop per year
20,000 lost articles of mail per hourSeven lost articles of mail per hour
Unsafe drinking water almost 15 minutes each dayOne minute of unsafe drinking water every seven months
5,000 incorrect surgical operations per week1.7 incorrect surgical operations per week
2 short or long landings at most major airports dailyOne short or long landing at most major airports every five years
200,000 wrong drug prescriptions each year68 wrong drug prescriptions each year

Key Features of Six Sigma

The key features of six sigma and the process improvement possibility after implementation, vis.-à-vis. classical approach to quality improvement, is that six sigma focuses on:

To efficiently drive improvements, the focus must be on the inputs (Xs) to the process. The word sigma is used in two ways, to describe capability and to describe variation. As DPMO goes down, process capability goes up.

How is Six Sigma different from other initiatives?


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About the Author

Rajeev Kumar
CEO, Computer Solutions
Jamshedpur, India

Rajeev Kumar is the primary author of How2Lab. He is a B.Tech. from IIT Kanpur with several years of experience in IT education and Software development. He has taught a wide spectrum of people including fresh young talents, students of XLRI, industry professionals, and govt. officials.

Rajeev has founded Computer Solutions & WebServicesWorldwide.com, and has hands-on experience of building variety of web applications and portals, that include - SAAS based ERP & e-commerce systems, independent B2B, B2C, Matrimonial & Job portals, and many more.

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