Six Sigma is a concept for quality management and process improvement based on quantitative, statistical analyses of key quality indicators. The name derives from the standard deviation of a normal statistical distribution, denoted by the Greek letter sigma. Six Sigma (i.e. six times the standard deviation) means that only 3.4 out of a million values are outside a specified tolerance. The focus is therefore on extreme quality requirements that can only be applied to quantity-intensive processes. The concept also includes specific procedures for process improvement and product development.