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Six Sigma Quality for Business and Manufacture

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The phrase "Six Sigma" derives from assuming a Normal distribution of some random variable that you are measuring. This distribution has a standard deviation, sigma. Then, if you measure the area under the curve that is greater than 6*sigma from the mean, you get something like under 10^-6. So the idea is that if you make something with a property that can vary from item to item, you want to reduce the deviation so that all items within 6*sigma of the mean are acceptable. And thus, unacceptable items are less than one in a million.

Of course, if the distribution is not Normal, then this is not necessarily true. In this case, Six Sigma is used metaphorically. The basic idea is the same.

The Six Sigma approach has been widely publicised in recent years. Perhaps the best known proponent in the United States is Motorola, who claimed that they successfully applied it to their manufacturing operations. Especially those for cellphones and microprocessors.

Perhaps its greatest merit is that it stresses hard numerical criteria that are easily and economically measurable. So that you can conduct these measurements on a large subset of the items you are making. Or indeed perhaps on all the items.