Six Sigma is A Performance Target

In their book Six Sigma:  The Breakthrough Management Strategy Revolutionizing the World’s Top Corporations, the authors Ikil Harry, Ph.D., and Richard Schroeder describe what Six Sigma is, namely a performance target, but they first disabuse the reader from some common notions that exist about what Six Sigma is.

False Notion #1:   Six Sigma describes an organization.   To say such-and-such company is a Six Sigma company may be misleading.   It probably means that the company’s industrial processes and commercial processes are operating at the Six Sigma level.

False Notion #2:   Six Sigma describes a product.   To say such such-and-such product is a six Sigma product may imply that it is relatively free of defects.

However, to use the most narrow, technically correct definition, Six Sigma ultimately specifies a performance target for a single critical-to-quality characteristic of a product.   It is stated in terms of “defects per million opportunities”.   What does the word opportunities mean in this context?    Basically, the more complex a product is, the greater the likelihood a defect will exist somewhere within the product because a) the product will contain more parts that in themselves may contain defects, and b) the parts of the product will be connected to each other in ways that may be defective.   So rather than saying an automobile is Six Sigma, it is technically more correct to say the average opportunity for a defect within a product is Six Sigma.

What is the difference between three Sigma and six Sigma?   In a terrific analogy, let’s say your house was flooded and the receding flood waters left mud on the carpet of your 1,500-square-foot home.   If the company you hired to clean the carpet operated at a three Sigma level, then it might leave as much as a four square-foot space in the carpet covered with mud–something bound to be noticed.  However, at the six Sigma level, the area of the carpet covered with mud would only be the size of a pinhead, and virtually invisible.

In another analogy that hits closer to home for business travelers, the airline industry’s record in getting passengers safely from one city to another exceeds Six sigma, but its record in getting those passengers’ bags safely to the same destination is closer to between 3.5 to 4 sigma (i.e., between 0.6 and 2.3% defect rate).   This is the difference between the high Sigma level of a complex manufacturing process that creates the airplane itself and the service operation that transports the baggage carried in the airplane.

The Sigma level affects the company’s bottom line because it effects the cost of quality that the company spends in its manufacturing or industrial processes.   The next post discusses this topic, which once understood, explains why in turn the Sigma level of a company’s processes is of such vital importance.


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