Six Sigma vs. Three Sigma

Instructor: Elizabeth Wamicha

Elizabeth has been teaching undergraduate courses in Business and Information Technology for the last 7 years. She is currently on course to completing a Doctorate in Information Systems

This lesson looks at two distinct methods of quality management which are Six Sigma and Three Sigma. The lesson examines the similarities and differences between the two methods as well as their application areas.

Six Sigma and Three Sigma as statistical approaches to quality management

The use of statistical methods for quality management and control was first developed by Walter Shewart in the 19th Century. These statistical methods formed the basis of Three Sigma and Six Sigma which are ideally two different levels of Sigma. The next section further analyses the Sigma levels.

Sigma Levels

Science of statistics, as developed by Shewart, relates to quality control metrics which state that exceptions or error rates found in a production process are empirical qualities that are based on a predetermined standard deviation.

For example, if a factory is producing wheels with a particular radius, diameter, and circumference measurements, then the error rates would be the extent to which wheels manufactured deviate from these predetermined measurements. Each Sigma level has a different tolerance level for errors.

Therefore, the Sigma One to Sigma Six levels describes the maximum number of defects per million in a system or process. There is, therefore, a strong relation between this and the overall accuracy percentage expected from the system or process. For example, the Three Sigma approach expects a maximum of 66.8K errors per million. This translates to 93.3% accuracy expectation for any given process or system. On the other hand the Six Sigma expects a maximum of 3.4 errors per million. This translates to 99.999997% accuracy expectation for a given process or system.

Six Sigma Overview

Six Sigma strives towards achieving near perfection for a given process. Therefore, from a practical perspective, Six Sigma attempts to continually improve processes and rate of accuracy such that it tends towards a perfect outcome. In this regard, Six Sigma does not only focus on setting up a measuring scale for performance but also provides a methodology that facilitates performance improvement. This is because Six Sigma addresses other factors including culture, environmental change and the human aspect as factors that contribute significantly to quality. Six Sigma attempts to provide a holistic approach to quality measurement of processes and symptoms and also quality improvement of those processes and systems.

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