Six Sigma is a quality approach used to strive for zero
defects in production. It is used as a management strategy for initiating
comprehensive reviews of products and processes. The goal of Six Sigma is to
reduce the defects to a maximum of 3.4 defects per million opportunities (dpmo),
or 99.99966 percent acceptance. Since it is not possible to achieve a defect
free process in the real world, Six Sigma is used to set a high standard for
measuring quality performance. By reducing the number of defects, or total
defects per unit (DPU), it becomes possible to produce more accurate products
and, therefore, improve producibility.
The term sigma is a statistical term that means standard
deviation. With Six Sigma, the total DPU is translated into a standard deviation
value. The sigma value indicates how often defects are likely to occur. The
higher the sigma value, the less likely a process will produce defects. A
company that has successfully implemented Six Sigma will spend one percent, or
less, of each sales dollar on the cost of non-conformance. Most companies in the
U.S. industrial base operate near the four sigma level and spend as much as 25
percent of each sales dollar on the cost of non-conformance.
If process capabilities are known, the methods and tools provided by the Six Sigma approach can help an organization understand, predict, and avoid the occurrence of defects in its products while they are still in the design phase of the product development process. As stated above, the total DPU output of a process is converted to a
standard deviation. Process capability is expressed as the capability index, which compares the output of the
process (DPUs converted to standard deviation) to that of the process tolerance. The process tolerance is defined as six standard deviations, or Six Sigma. There are two capability indexes used to define process capabilities. Cp is the capability index of the nominal target process. It is the ratio of design tolerance to Six Sigma process variability.
Cpk is Cp adjusted for the difference between the nominal
process mean and the actual process mean.
For an organization determined to raise its standard of
acceptable products and processes, use of the Six Sigma approach is an excellent
means for accomplishing this goal. The benefits of incorporating this
methodology include: improved production cycle times; reduction in errors,
rework, and scrap; and gains in productivity. These benefits translate into
improved product performance and reliability as well as lower product
Although the process of Six Sigma can be involved, the
payoffs to producibility are significant. There are numerous training courses,
software tools, reference materials, and consultants available to assist in its
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Massy, M. J. (1992). Six Sigma Producibility Analysis and Process Characterization. Massachusetts: Addison-Wesley.
Perez-Wilson, M. (1999). Six Sigma: Understanding the Concept, Implications and Challenges. Advanced Systems Consultants.