Six Sigma is a comprehensive approach to organizational change that is based on behavioral concepts and the use of statistical information to aid in decision making. It evolved from the principles of Total Quality Management, an earlier method designed to change organizations. The name “Six Sigma” is derived from the statistical properties of the normal distribution, presented in Chapter 2. Sigma is the term associated with the standard deviation, the statistical index that reveals the spread or variability of scores in a distribution. Plus or minus one sigma (standard deviation) encompasses approximately the middle 68% of the scores in a normal distribution, plus or minus two sigmas encompasses approximately 95% of the scores, and plus or minus three sigmas encompasses
approximately 100% of the scores in a distribution (see Figure 2-5a).