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Chapter 1 : Creating a Bayesian network (modeling mode)
BayesiaLab's graphic interface allows for an intuitive, manual development of a bayesian network. In order to access this functionality, either a new graph must be created or an existing graph must be edited.
Once a worksheet is open, new icons appear in the toolbar:
- Create a new graph
- Open a graph («xbl», «bif» or «net» format)
- Save a graph
- Print a graph
- Cut
- Copy
- Paste
- Cancel the last action
- Redo the last action
- Find a node
- Zoom in
- Zoom out
- View by default
- Adjust position on the page
- - Select a node or an arc
- - Create a node
- - Create a constraint node
- - Create a utility node
- - Create a decision node
- - Create an arc
- - Delete an arc or a node
1.1 Creating nodes and arcs
First, we'll consider the case of a specialist of lung illnesses who wants to model his or her knowledge (very simplified) concerning a diagnosis of cancer or tuberculosis.
The development of a Bayesian network relies on using the previously collected variables needed for describing the domain under consideration. Each of these variables then corresponds to a node in the graph.
Using the button "create a node" (or by holding down the "N" key while performing a left click), our specialist adds three nodes to the worksheet: one representing the age of the patient, another one to represent if the patient smokes or not and the last one to represent if he or she has cancer.
A warning icon is displayed at the top left of a node to mean that the associated probabilities were not checked or are incorrect. To display the warning message, simply move the mouse cursor over the node maintaining the key W pressed.
He defines then the probabilistic relations between these variables. These relations are shown by arcs that can be created with the "create an arc" button, by moving the mouse, with the left button held down, from one node to the other (it is also possible to switch to the "create an arc" mode by holding down the "L" key while pulling the arc).
- The age has a direct influence on Smoker and Cancer
- Smoking influences cancer
The screenshot given below corresponds to the contextual menu associated to the nodes in Modeling mode (the gray items are only available in Validation mode, which means that they are associated to inference). Whereas the first function is used for editing the properties of each node (cf. chapter 1.2), the definition of a target node is especially useful for automatically learning a Bayesian network from a data base (cf. chapter 3). The «Node Tagging» item allows tagging the nodes with specific colors (to locate quickly for example the nodes that represent symptoms, diseases