MidpointWhat is Six Sigma?
Sigma is a Greek letter that has been used by mathematicians for many years to describe a variation from what is expected. It is more commonly known as the “standard deviation” and it is used in statistics to describe the difference of an observation, from the normal or expected value. Many things in life follow a “normal” distribution, and most people are familiar with this as a bell curve.
Midpoint
The normal distribution has a symmetrical pattern about the midpoint. The area of the curve shown in red indicates the amount of the distribution that is covered by 1 standard deviation (or sigma) above and below the midpoint. This will cover 68.26% of the total distribution. If we expand the range to 2 standard deviations (or 2 sigma) the coverage area will include the red and green areas, and the area will include 95.46% of the total distribution. This continues until we get to 6 standard deviations, and now the coverage area is expanded to include 99.99997% of the total distribution. A company that follows a 6 sigma program, is striving to achieve less than 3.42 defects per million opportunities.
You may think that it is difficult to measure a defect level of 3.42 per million when you are building an individual product like a boat, but just think how many ways a boat can have a defect, there are thousands of opportunities for mistakes every time a single item is built.
Several companies have embraced the Six Sigma program with great success. Notable examples include General Electric, Motorola, Allied Signal, Microsoft, Bank of America etc, and they have achieved a quantum leap in their quality because they have shifted their focus from defect detection to defect prevention, and they have switched measurements from scrap rate expressed as % into part per million defect levels.