Six Sigma refers to a set of tools and techniques, within an improvement cycle, developed by Motorola to guide the process of continuous improvement of product quality. The term itself refers to a level of process capability. Sigma (s) is the Greek alphabet equivalent of the letter s, and is used to denote standard deviation in statistics. Any group of values resulting from a numerically-based process measurement activity will have an average or mean value (s) denoting its 'centre', and a standard deviation (s) denoting its degree of inherent variability. Standard deviation is itself a measure of the accumulated differences between a group of values and their average. Traditional quality control theory has always held that having the upper or lower specification limits for a product or component equate to three standard deviations from the mean for the process would result in acceptably low levels of nonconforming product (66,807 defects per million opportunities, or 6.7% of production, assuming that processes are subject to disturbances that could cause the process mean to shift by as much as 1.5 standard deviations off the target). In Six Sigma thinking, the process is improved to the extent that specification limits lie six standard deviations from the process mean, and thereby achieve 3.4 defects per million opportunities (or 0.00034% of production). Figure 1 compares the two situations graphically. The means to achieving this level of performance are through application of a collection of long standing management and statistical tools, systemically applied within a cycle of improvement known as DMAIC (standing for Define - Measure - Analyse - Improve - Control).