Selecting a supplier by fuzzy evaluation of capability indices C pm

A process capability index C_pm that fits nominal-the-best type quality characteristics is an effective tool for assessing process capability since the index can adequately reflect a centring process capability and process yield. A valuable method using C_p estimators was developed by Chou 7] for practitioners to use to determine whether or not two processes have equal capability. However index C_p failed to measure process yield and process centring with bilateral specifications and in addition, more than two suppliers can be selected in an actual application. This study proposes a fuzzy inference method to select the best among the competing suppliers based on an estimated capability index of C_pm calculated from sampled data. This method has the advantages of fuzzy systems where a grade can be obtained instead of a more specific exact evaluation result. An illustrated example of colour STN displays demonstrates that the proposed method is effective and feasible for the evaluation of competing process capability.