Process Yield and Capability Indices

Capability indices were introduced to allow comparing the performance of various processes independently of their tolerance interval. The concept of performance, related first to the proportion of conforming items (process yield), quickly evolved to also take into account the process position compared to its target (process centering). Many indices were thus suggested, and the families Cp(u, v) in the case of symmetrical tolerances, then [image omitted] in the case of asymmetrical tolerances, allowed to generalize the most usual indices. If the links between capability indices and process centering were studied, those between capability indices and processes yield were studied only partially. In this article we clarify the links between the process yield and the indices [image omitted] for all the positive or null values of u and v.