352 14 Bayesian Networks
section 14.3.4. The authors have studied the use of their methodology within
the domain of esophageal cancer.
The Helsper-van der Gaag methodology uses ontologies more as a back-
ground for the design process than as a formal specification for the BN struc-
ture. This is in contrast with the OOBN technique in subsection 14.3.3 in
which the design not only specifies the BN completely but also affects the
inference algorithm. In the Helsper methodology the ontology is used to pro-
vide an initial design for the BN in a manner similar to the way that this is
done in subsection 14.3.4. This step in the methodology is calledtranslating.
However, this initial design is modified in a series of steps based on domain
knowledge. Some of the modifications use the ontology, but most of them
must be elicited from domain experts. The ontology “serves to document
the elicited domain knowledge.”
What makes the Helsper-van der Gaag methodology interesting are the
systematic modification techniques that are employed. The methodology
refers to this phase asimproving and optimizing. The modifications must fol-
low a set ofguidelines, but these guidelines are only explained by examples
in the articles.
One example of a modification operation used by Helsper and van der
Gaag is shown in figure 14.7. In this operation, a node that depends on two
(or more) other nodes is eliminated. This would be done if the node being
eliminated is not observable or if it is difficult to observe the node. There
are techniques for determining the CPDs for unobservable nodes such as
the EM algorithm discussed in subsection 14.3.2. However, this algorithm is
time-consuming. Furthermore, there is virtually no limit to what one could
potentially model, as discussed in section 13.3. One must make choices about
what variables are relevant, even when they could be observed, in order to
make the model tractable.Figure 14.7 Modifying a BN by eliminating a node that other nodes depend on. The
result is that the parent nodes become dependent on each other.