ABSTRACT
The Cpm index of process capability was introduced in the published literature by Chan et al. An elementary discussion of the index, written for practitioners, was presented by Spiring, who also extended the application of the index to measure process capability in the presence of 'assignable causes'. He had described the application of Cpm to a toolwear problem, as an example of a process in which there was variation due to assignable causes. The use of the Cpm index was proposed by T.C. Hsiang and G.Taguchi at the American Statistical Association's Annual Meeting in Las Vegas, Nevada. Proponents of Hsiang and Taguchi's approach point out that it does allow for differing costs arising from different values of X in the range LSL to USL, though in a necessarily arbitrary way. The index Cpm attempts to represent a combination of effects of greater variability and of greater relative deviation of ξ from the target value.
