Six Sigma: Controlling Variation

Reducing Variance using Statistical methods

Six Sigma is a registered trademark and service mark of Motorola, Inc.

What is Six Sigma

Originally, Six Sigma is a methodology aimed at recognizing, analyzing and solving of (too) many variations in processes. Each process has its variations. They are the cause of a lot of internal deficits and complaints by customers. Six Sigma offers a methodology to minimize this variation in process.
Over time, the Six Sigma methodology has grown to a collective term of all kinds of techniques and methodologies with the goal of minimizing the defaults per million actions. Within Six Sigma, the word process contains more then only the production process. Also the development process, sales and purchasing can be improved with Six Sigma.

Six Sigma: In the beginning

 From 1985 onwards, Six Sigma (6s) has been developed by the Motorola Company from the US. The definition was as follows:
“A methodology of defining, identifying, measuring, analyzing, improving, and controlling of products and processes.”
(F. Stevens)

General Electrics picked up Six Sigma and developed it further. GE describes it as follows:

“The central idea behind Six Sigma is that if you can measure how many defects you have had during a process, you can systematically figure out how to eliminate them and get as close to “zero defects” as possible.”
(What is Six Sigma?, the Roadmap to Customer Impact, GE)

In each process, it is inevitable that defects occur. The number of mistakes can be regarded as a quality indicator. In general, the Six Sigma Roadmap is used to improve that quality.

Step 3 measures how many defects occur during each step of the process. The sum of all defect percentages defines the probability of a defect to occur. The specialty of Six Sigma is that the company defines a standard for the maximum allowable probability for a defect to occur (process variation). This probability can be rated as follows:

Sigma factor

Number of defects per million

Quality grade (yield)


690000 dpm

30,9 %


308000 dpm

69,2 %


66800 dpm

93,3 %


6210 dpm

99,4 %


230 dpm

99,98 %


3,4 dpm

99,9997 %

Sigma Capability

The average level of American companies is 3 to 4 Sigma. Thanks to the Six Sigma program with Motorola, quality improved over a period of three years from 3 to 5,7 sigma. The original aim of Six Sigma was to reduce process variations.

Internally, Six Sigma uses an improvement methodology that consists of five steps. The DMAIC-circle visualizes this methodology. DMAIC means Define, Measure, Analyze, Improve and Control. There exists a clear analogy with the Plan-Do-Check-Act-circle of Deming, which, in turn, is the basis for SGA-circle used by Blom Consultancy.

SGA circle

Just like with other WCM-programs, the approach to problems is generally by multifunctional teams. In this respect, the idea is that by combining different talents, cultures and functional professionalism you can obtain a bigger problem solving capacity.

Six Sigma can be subdivided into six themes:

  • Customer focus
  • Facts and Figures
  • Process focus, management and improvement
  • Pro-active management
  • Boundary less Collaboration (a term coming from Jack Welch) = go for it
  • Strive for perfection (Zero waste, zero defects)

Six Sigma: What did become of it?

Over time, several companies started to use Six Sigma to improve processes. For a long time, it has not only been restricted to production processes, but one is also using the same methodologies to tackle logistical, administrative, and other processes.

In the meantime, Six Sigma became a collective term for all kinds of techniques and methodologies, supporting that the company is going to be a world-class company. The aim still is to reduce variations in process; however, each technique related to the DMAIC-circle and can make a positive contribution is used. An unambiguous Six Sigma definition is not used any more. Several definitions are in circulation.

Peter S. Pande formulated the modern Six Sigma definition as follows:

“A comprehensive and flexible system for achieving, sustaining and maximizing business success. Six Sigma is uniquely driven by a close understanding of the needs of the customer, disciplined use of facts, data and statistical analysis and paying close attention to managing, improving and reinventing business processes.”
(The Six Sigma Way, Peter S. Pande et al, McGraw-Hill: 2000)

More information

Author: Bert Teeuwen, Blom Consultancy

© 1996-2021 Arno Koch. Please contact Arno Koch if you want to use content of this site!