Some problems and studies on process capability indices

According to the results of some surveys, most of companies have been using the C-p, C-pk, C-pm and other indices. Process Capability Indices (PCIs) are an important part of SPC and standard tools in Six Sigma programs. These indices are utilized in estimating the capability of a production process. The CP is "basic" PCI, and some other "advanced" indices, for example, C-pk, C-pm, C-pmk and etc., are employed, the core of which is the emendatory or non-emendatory ratio of the span of upper and lower specification limits designed to the span of 6sigma in normal distribution of the datum measured. Based on the value of the indices, the process capability can be estimated. Cp and Cpk are related to the expected proportion, the Cpm is related to loss function instead. C-pmk is a combination of the two of them. Cp reflects the inherent variation of process. Cpk reflects the total variation of the inherence and outside influence. Cpm reflects further the relative of the datum to target value. This paper systematically analyzes their difference and compares practical utility of these indices, so as to find the applicability of these indices. Some problems in practical usage of PCIs are discussed in the paper