The Quality Loss Index QLI and Its Properties

This article investigates the quality loss index QLI proposed by Samajdwar et al. for normal variables. The probability distribution of QLI and its components are obtained. The first component represents a quality loss index for the process variability, while the second component represents a quality loss index for the shift in the process mean. Moreover, a method is derived allowing to estimate the nonconformance probability in either side of the target value for normal variables. Finally, an additive property of the quality loss index is established, which enables to define an composite quality loss index and use it within a continual process improvement strategy. The composite quality loss index indicates the process performance and identifies the parameter(s) which show a potential for improvement.