Qualitative expressions of changes

Qualitative expressions of change in signals, such as gradually increasing, smoothly changing, suddenly goes up, and swing quickly, are often used in diagnostic and control knowledge acquired from humans. The knowledge is then translated into rules for knowledge-based systems. These expressions, in general, can be directly expressed neither by qualitative reasoning methods nor by fuzzy techniques. The author defines these expressions linguistically using terms of magnitude such as high and small. By defining these magnitudes with fuzzy sets or with landmark values, the change expressions can be indirectly defined. This indirect definition allows flexible application of the qualitative expressions whose meaning heavily depends on the context and situations when these expressions are used