“…However, the design of the diagnostic strategy is often made di cult by the complexity of the system, which can limit the use of quantitative model-based approaches requiring a very detailed and well-tuned analytical model of the whole plant, such as the methods described in Gertler (1998), Patton et al (1989), Isermann (1993), Park et al (1994) and Kinnaert (1999). Techniques relying on more qualitative models, such as those based on arti®cial intelligence (Kleer et al 1992, Struss et al 1996, fault trees (De Vries 1990, Lee et al 1985, Pages and Gondran 1986 or propagation digraphs (Iri et al 1979, Kokawa et al 1983, Koscielny 1995, Scattolini 2000, usually do not possess the characteristics of modularity and¯exibility that play a crucial role in the development and maintenance of an e ective design methodology widely applicable in an industrial environment.…”