6-Sigma refers to a quality improvement and business strategy concept started by Motorola in the United States in 1987.  In statistical terms, 6-Sigma is the abbreviated form of 6 standard deviations from the mean, which mathematically translates to about 2 defects per billion.  Thus, strictly speaking, your process is said to have achieved 6-sigma if it is producing no more than 2 defects per billion parts produced.


No company is probably nearly perfect enough to achieve this quality level.  Consequently, the term 6-Sigma in the industry has somehow taken on the equivalent defect rate of 3.4 ppm, which in reality corresponds to roughly 4.5 sigmas.  Thus, in the industry today, a person speaking of 6-sigma is most likely referring to a quality level equivalent to 3.4 defects per million.


Regardless of how one wishes to use the term 6-sigma, though, it is apparent that its purpose when its concept was first incepted is to make processes as consistent as possible in order to reduce the defect rates of their outputs. Consistency of meeting customer specifications as well as the probability of meeting them consistently in the future is the essence of 6-sigma.  To see how the number of sigmas relates to the process Cpk and the process ppm level, please refer to the Cpk/ppm Table.


6-Sigma has evolved into a continuous, disciplined, and structured process of improving operations to make products that are consistently meeting customer requirements. In effect, 6-Sigma no longer simply means excellent finished products, but more importantly, excellent processes, services, and administration.  When Motorola started 6-Sigma in the 80's, it was applied to repetitive manufacturing processes.  Presently, however, the use of 6-Sigma is well-established in almost all aspects of doing business in a wide range of industries.      


6-Sigma encourages leanness, simplicity, and doing things right the first time, so that wastes and corresponding costs are avoided.  Statistics-based problem solving, results-orientation, and quantifiable top and bottom-line returns are also ingredients of 6-Sigma.  Lastly, 6-Sigma is driven by the voice of the customer.


6-Sigma has spawned several Project Management methods, the most widely-used of which are discussed below.


Define, Measure, Analyze, Improve, and Control (DMAIC)


'DMAIC' stands for the following:

1) Define opportunities, i.e., project goals in relation to customer requirements;

2) Measure the current performance of the process;

3) Analyze the weakness of the process (such as sources of defects); this process weakness is also the opportunity for its improvement;

4) Improve the performance of the process by addressing its weaknesses; and

5) Control the performance of the improved process to sustain its gains.


The DMAIC method is employed in situations wherein a product or process already exists but it is not meeting customer specifications.


Design for Six Sigma (DFSS)


'DFSS' is the acronym for Design for Six Sigma.  Unlike, the DMAIC, there is no single or standard definition of what steps or phases the DFSS process consists of.  It is generally up to the company to define the steps needed to design its processes to be capable of 6-sigma quality level, i.e., 3.4 ppm.  DFSS may therefore be customized to the nature of business and culture of the practicing company.  DFSS is generally used when designing a new product or completely redesigning an existing one from scratch.


Define, Measure, Analyze, Design, and Verify (DMADV)


'DMADV' stands for  the following:


1) Define opportunities, i.e., project goals in relation to customer requirements;

2) Measure and determine customer requirements and how competitors are serving these requirements;

3) Analyze your process options to meet these customer needs;

4) Design your process to meet these customer needs; and

5) Verify the performance of the process, particularly in terms of its ability to meet customer requirements.


The DMADV method is employed in situations wherein there is no existing process or product yet catering to a certain customer requirement, and the company wants to develop one for that purpose.


See Also:   Lean ManufacturingTQMTPMKaizenSPC5S Process;   Poka-Yoke




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