  Relational models for diagnosis are based on a direct description of the
association between disorders and manifestations. This type of model has been
specially used and developed by Reggia and his co-workers in the late eighties
as a basic starting point for approaching diagnosis problems. The paper
proposes a new relational model which includes Reggia's model as a particular
case and which allows for a more expressive representation of the observations
and of the manifestations associated with disorders. The model distinguishes,
i) between manifestations which are certainly absent and those which are not
(yet) observed, and ii) between manifestations which cannot be caused by a
given disorder and manifestations for which we do not know if they can or
cannot be caused by this disorder. This new model, which can handle uncertainty
in a non-probabilistic way, is based on possibility theory and so-called
twofold fuzzy sets, previously introduced by the authors.
