A SMOOTH NONPARAMETRIC APPROACH TO PROCESS CAPABILITY

The determination of the capability of a stable process using the stan dard process capability indices requires that the quality characterist ic of interest be normally distributed. Departures from normality can result in erroneous conclusions when using these indices. In this pape r we propose assessing the capability of a process using a nonparametr ic estimator. This estimator is based on kernel estimation of the dist ribution function. Bandwidth selection for this method can be based ei ther on a normal reference distribution or on a nonparametric estimate . An example is presented that applies this proposed method to non-nor mal process data. The performance of the resulting estimator is then c ompared with the sample proportion and a normal-based estimate in a si mulation study. (C) 1998 John Wiley & Sons, Ltd.