First generation expert systems were using shallow knowledge based on heuri
stic information to solve a diagnostic problem. This approach has many disa
dvantages, which can be avoided by using deep knowledge. Diagnostic reasoni
ng based on deep knowledge is called model-based diagnostics. Recently, the
use of qualitative modeling in relation to deep knowledge in expert system
s has become increasingly important. The main purpose of our contribution i
s to present the model-based diagnostic approach at a formal level. The ori
ginality of the presented formalization is the concept of the diagnostic sp
ace, the characterization of the minimal diagnoses, and the measurement. Th
e formalization serves as the theoretical background to prove our view to t
he design of qualitative system models and to establish the diagnostic arch
itecture called DISY. The qualitative system model in our diagnostic approa
ch needs not to be specially adopted for use in the diagnostic domain. The
only requirement is that it must simulate the system behavior expressed by
normal or abnormal functioning of its components. Proposed DISY architectur
e is not complex and simply takes into an account the previous diagnostic r
esult to obtain a new one from the additional observation-measurement (medi
cal tests or examinations) of the system. (C) 2001 Elsevier Science Ireland
Ltd. All rights reserved.