One‐sided capability indices C_PU and C_PL: decision making with sample information

Process capability indices have been used in the manufacturing industry to provide quantitative measures on process potential and performance. The formulae for these indices are easy to understand and straightforward to apply. But, since sample data must be collected in order to calculate these indices, a great degree of uncertainty may be introduced into capability assessments due to sampling errors. Currently, most practitioners simply look at the value of the index calculated from the sample data and then make a conclusion on whether their processes meet the capability requirement. This approach is not reliable since sampling errors are ignored. Procedures for two‐sided capability indices, C_p, C_pk, and C_pm have been developed to assist practitioners to determine whether their processes meet the capability requirement based on sample information. In this paper, we first obtain unbiased estimators of C_PU and C_PL. We then develop a procedure similar to those of C_p, C_pk, and C_pm, for the one‐sided capability indices C_PU and C_PL. Practitioners can use the procedure to test whether their processes meet the capability requirement.