# Research

This page describes some of my research interests; for all the details you will have to take a look at my publications or presentations.

Way back, when I had to choose a topic for my MSc-thesis project, I already was very interested in Bayesian networks. Nowadays, Bayesian networks and related models are often called a probabilistic graphical model.

## QPNs and verbal probability elicitation

For my MSc-thesis project I investiged *qualitative probabilistic networks (QPNs)* and proved some of their
properties. Qualitative probabilistic networks are basically abstractions of Bayesian networks, where the
conditional probability tables for the variables are replaced with signs of influences between variables.

For my PhD research I considered qualitative approaches to quantifying Bayesian networks. Since QPNs can be used as an intermediate step in the construction of Bayesian networks, we can benefit from the use of QPNs most if they are a powerful as possible. We therefore extended the basic framework in a number of ways; see for more details my summary of QPN research. In addition, I focussed on methods for eliciting probabilities from domain experts. More specifically, we designed a verbal-numerical probability scale which enabled us to elicit the thousands of probabilities we required for a network on oesophageal cancer.

## Construction of probabilistic networks

The research described above on QPNs and probability elicitation can be considered as research related to the
construction of the quantitative part of probabilistic networks. Although still interested in these topics,
my interests concerning the quantification of probabilistic networks have broadened. My main research topic
in this area has been that of *sensitivity analysis*. Sensitivity analysis is a general technique for
studying the relationship between the input parameters and the output of a mathematical model. Sensitivity analysis
is a useful tool during quantification: you can use a fast and inaccurate method to elicit probabilities, and then use sensitivity analysis to investigate which parameters are strongly affecting your output, and spend more effort on getting those accurate. Sensitivity analysis can also be used to tune the parameters of your network, and study the effects of for example removing an arc from the network.

Other interests include the study of monotonicity properties of Bayesian networks.

## Analysis of network behaviour

The theory behind sensitivity analysis can also be used to study properties of probabilistic models and conclude something about their general behaviour. In addition, sensitivity analysis can be used to study the robustness of a network after complete construction.

Other interests include the study of performance measures for probabilistic networks.