Statistical quality control based on the process capability index and control charts with fuzzy approach (Case study: water and wastewater company of West Azarbaijan province)

Statistical quality control is a method for monitoring the process to identify the underlying causes of changes and carrying out corrective actions. Process and capability control charts are two important applied tools for statistical quality control. In many actual systems in which accurate and certain information is not always available and the information is vague and fuzzy, fuzzy based methods can survey production process more precisely using appropriate linguistic terms and fuzzy numbers. In this study, fuzzy control charts are developed using fuzzy rules, and then the fuzzy actual capability index of process ( ) is investigated to evaluate the precision, accuracy and performance of production process in the fuzzy state. The results of the study on the quality of water flowmeters in the urban water and wastewater company of West Azarbaijan province showed that using fuzzy rules compared to the crisp data, provides more decision-making options to decision- makers and provides more precise division about the product quality. Also, the fuzzy actual capability index of process can propose a more precise analysis of the process taking into account the average, target value and process variance, simultaneously. The values of the fuzzy actual capability index of process in the studied case are less than one, showing that the conditions of the production process are unfavorable.