A practical implementation of the incapability index C-pp

Abstract

Process capability index C-pp has been introduced to measure process performance. In practice, since sample data must be collected in order to calculate the index value, then a great degree of uncertainty may be introduced into capability assessments due to sampling errors. In current practice, the practitioners would simply look at the index value calculated from the sample data then make a conclusion on whether the given process meets the capability requirement. This approach is not reliable since the sampling errors are ignored. In this paper, we take into account the sampling errors and develop a simple procedure using the UMVUE (uniformly minimum variance unbiased estimator) of C-pp, for practitioners to use in determining whether a process meets the capability requirement. The decisions made based on the proposed procedure are, of course, more reliable.