12. Sigma Level Estimation

The sigma level estimation sets the base for improvement for any process. After setting the goal, the Six Sigma process performance is evaluated in terms of its sigma level, and for the evaluation of sigma level, we need to have the knowledge of the specification limits and target of the process. The process performance is also seen in terms of its long-term and short-term performance. All this concepts are discussed in this chapter. The sigma level is usually calculated on the basis of process capability, which generally depends on the type of data under investigation. For attribute data system, the capability is calculated in terms of defects per million opportunities often called parts per million, whereas for continuous data system, the capability is defined in terms of defects under the curve and outside of the specification limits. An important performance dimension not captured by defects or sigma level is the cost of impact of defects, often called cost of poor quality (CoPQ). The CoPQ quantifies the money lost as the result of defects and problems. It is essentially the cost of the defect or problem that has been identified in the process. It also refers to the overall cost of whatever defects are present in the process. Hence, various types of costs associated with quality are taken up for discussion in this chapter.