6-Sigma

6-Sigma refers to a quality improvement and business strategy concept started by Motorola in the United States in 1987.  In statistical terms, 6-Sigma is the abbreviated form of 6 standard deviations from the mean, which mathematically translates to about 2 defects per billion.  Thus, strictly speaking, your process is said to have achieved 6-sigma if it is producing no more than 2 defects per billion parts produced. 

No company is probably nearly perfect enough to achieve this quality level.  Consequently, the term 6-Sigma in the industry has somehow taken on the equivalent defect rate of 3.4 ppm, which in reality corresponds to roughly 4.5 sigmas.  Thus, in the industry today, a person speaking of 6-sigma is most likely referring to a quality level equivalent to 3.4 defects per million.

Regardless of how one wishes to use the term 6-sigma, though, it is apparent that its purpose when its concept was first incepted is to make processes as consistent as possible in order to reduce the defect rates of their outputs. Consistency of meeting customer specifications as well as the probability of meeting them consistently in the future is the essence of 6-sigma.  To see how the number of sigmas relates to the process Cpk and the process ppm level, please refer to the Cpk/ppm Table.

6-Sigma has evolved into a continuous, disciplined, and structured process of improving operations to make products that are consistently meeting customer requirements. In effect, 6-Sigma no longer simply means excellent finished products, but more importantly, excellent processes, services, and administration.  When Motorola started 6-Sigma in the 80's, it was applied to repetitive manufacturing processes.  Presently, however, the use of 6-Sigma is well-established in almost all aspects of doing business in a wide range of industries.      

6-Sigma encourages leanness, simplicity, and doing things right the first time, so that wastes and corresponding costs are avoided.  Statistics-based problem solving, results-orientation, and quantifiable top and bottom-line returns are also ingredients of 6-Sigma.  Lastly, 6-Sigma is driven by the voice of the customer.

6-Sigma has spawned several Project Management methods, the most widely-used of which are discussed below.

Define, Measure, Analyze, Improve, and Control (DMAIC)