7. A Fuzzy Handling of the Multi-criteria Characteristic of Manufacturing Processes

This chapter deals with the performance expression problematic in an industrial continuous improvement process. Performance expressions are the purpose of performance indicators and performance measurement systems (PMSs). We focus particularly on the elementary aspect of such an expression. The elementary performance expression is the constitutive element of the PMSs, being defined through the achievement degree of a considered objective, while other types of expressions are involved in PMSs, with regard to the multi-criteria and multilevel aspects of the objectives. The computation of the objective achievement brings together the objective declaration, the acquired measurement that reflects the reached state and the comparison of these parameters. By revisiting previous works handled in this field, we consider that elementary performance expression is modelled by a mathematical function that compares the objective to the measurement. Conventional Taylorian ratio and difference are highlighted. The qualitative or quantitative characteristic of the data, the flexibility concerning the objective declaration and the measurements errors lead us to use the fuzzy subset theory as a unified framework for expressing performance. It also leads to new approaches which are beyond comparison functions.