Fuzzy evaluation of process capability for bigger-the-best type products

Many industrial products can be characterised as of the bigger-the-best type. Quality characteristics and process yields of a bigger-the-best type product can be evaluated by using a process capability index. The process capability index Cpl is used to evaluate quality characteristics and process yields of the bigger-the-best type products because the formulae for this index are easy to understand and straightforward to apply. However, since sample data must be collected in order to calculate this index, a great degree of uncertainty may be introduced into the capability assessments due to sampling errors. The other shortcoming of the evaluation is that there exists a fuzzy condition for making a decision for rejecting a null hypothesis. In this paper, a method to incorporate the fuzzy inference with the process capability index in the bigger-the-best type quality characteristics assessments is presented, and a concise score concept is used to represent the grade of the process capability. An example shows that the proposed method is effective in application and thus confirms its feasibility.