Six Sigma Defined

Six Sigma is a data-driven, methodical program of continuous and breakthrough improvement focused on customers and their critical requirements. Sigma refers to the Greek symbol (s) that represents the amount of variation in a process. The lower the variation in a process, the fewer defective parts or service transactions are produced, and the higher the Sigma number. The ultimate goal is to eliminate defects and errors and the costs associated with poor quality. After defining which performance measures represent Critical to Customer (CTC) requirements, data are collected on the number of defects and then translated into a sigma number. A sigma of 6 translates to 3.4 defects per million opportunities. (See Table 1.) It is common to find 3 to 4 sigma levels in many manufacturing processes, and 2 or 3 sigma in transactional businesses. Moving from 3 to 4 sigma could be classified as continuous improvement. The breakthroughs occur when a process is improved to the 6 sigma level, almost perfect quality. For example, U.S. daily mail delivery at the 4 sigma level would result in the loss of 20,000 pieces of mail each hour. If mail delivery were at the 6 sigma level, the result would be the loss of seven pieces of mail each hour.