Six Sigma SPC - Statistical Process Control

Six Sigma SPC's Quality Control Dictionary and Glossary
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Six Sigma or 6 Sigma

Level of process performance equivalent to producing only 3.4 parts per million defects or has a yield of 99.9997%. (Also see RTY, (Rolled throughput yield)) Used to improve processes using a sigma based process measurement and/or striving for a six sigma level of performance. (See below or our page on the  original six sigma theories and concepts) Used in quality assurance to measure a quality level. Also used by management in various implementations to measure performance of services and the like.

Almost always related to a quality improvement plan and not quality assurance system. Can also be related to only cost improvement strategies. Developed by Motorola in the 1980's.

Using the theory that approximately 2700 parts per million, (PPM), parts/steps will fall outside the normal variation of +/- 3 Sigma.

In spite of this, when we build a products containing 1200 parts/steps, we can expect 3.24 defects per unit (1200 x .0027), on average. The result of this is a rolled yield of less than 4%. That is equivalent to fewer than 4 units out of every 100 will go through the complete manufacturing process without any kind of a defect.

Defects vs Rolled Throughput Yield

Therefore, for a product to be built "defect free", it must be designed to tolerate characteristics or parameter, which are significantly more than +/- 3 sigma away from the mean. (also see our page DFSS, design for six sigma)

It can be shown that a +/- 6 sigma can be expected to have no more than 3.4 parts per million defective for each characteristic, even if the process mean were to shift by as much as +/- 1.5 sigma. In the same case as above, a product containing 1200 parts/steps, we can now expect only only 0.0041 defects per unit (1200 x 0.0000034). This would mean that 996 units out of 1000 would go through the entire manufacturing process without a defect as the distribution below shows.

4 Sigma vs 6 Sigma Distribution

Both our software products are geared to this end. And at only $150.00 and $65.00, (USD), our software can get you going quickly.


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Last Updated: Thursday, 11-May-06 14:49:03 PDT