Effects of Shifts in the Process Level on the Taguchi Capability Index, Cpm, 1995 Process & Product Quality Conference Proceedings

Olle Carlsson,Ph.D.

University of Örebro

Sture Rydin, Ph.D

STORA CELL INDUSTRIES

An industrial process may well be centered on the target value but more or less permanent deviations are often unavoidable. They can be classified as: a systematic bias, assignable or common causes. Assignable causes could at least by some effort be identified and removed. Typical assignable causes (are for instance machine-to-machine differences, shift-to-shift effects and variations in the input raw material. By common causes we understand causes, which are hard or almost impossible to identify and to remove. These causes are considered as random noise. Steady state, or equivalently, the absence of assignable causes was earlier a necessary requirement for using capability indices in a meaningful way but in this study, the Taguchi index. cpm l . is generalized to allow the presence of assignab e causes. Small and large sample confidence intervals are given and demonstrated.