The robustness of two popular process capability ratios, Cp and Cpk, when the random process being observed departs from normality is analysed. The distributions of estimated process capability ratios are derived and used as a basis for validation of large-scale simulation studies in an examination of departures from normality. The simulation studies and analytical results provide a basis for recommended procedures. It is recommended that the impact of process distributions be considered before using popular process capability indexes, due to the lack of robustness when departures from normality are encountered. As an extension of the present findings, the authors consider the Taguchi loss function as a generalization to process capability analysis regardless of the underlying population distribution.
