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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:

  • the customer - critical to quality characteristics
  • data driven improvements
  • the inputs (Xs) of the process
  • reducing or eliminating defects
  • reducing variation
  • increasing process capability

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?

  • Six Sigma uses a structured approach towards Process Improvement with some defined tools.
  • Six Sigma gives us a Quantitative method to measure our process.
  • Six Sigma tells us where we are and where we need to go.
  • Six Sigma gives us a direct link between the dependent and independent variables.
  • By implementing Six Sigma we can control the mean and the spread about the mean as per the customer requirements.
  • Six Sigma answers How to improve our process rather than answering to What to do to improve our process
  • Six Sigma reduces the process variation thereby assuring the sustain of the benefits in the long term

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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 premier engineering colleges & management institutes, and IT professionals.

Rajeev has founded Computer Solutions & Web Services Worldwide. He has hands-on experience of building variety of websites and business applications, that include - SaaS based erp & e-commerce systems, and cloud deployed operations management software for health-care, manufacturing and other industries.

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