Building a (Limited Memory) Influence Diagram

This tutorial shows you how to implement a small influence diagram in the Hugin GUI. It requires that you have already constructed the Bayesian network from the first tutorial. The influence diagram you are about to implement is the one modeled in the introduction to influence diagrams in the getting started section. It helps plantation owner Apple Jack to decide whether or not to give his apple tree, which is losing its leaves, some treatment. The qualitative representation of the influence diagram is shown in figure 1.

Figure 1: The qualitative representation of the influence diagram used for decision making in Apple Jacks plantation

Open the BN for Editing

First, you must open the BN constructed in the first tutorial if it is not already open. Here is how to do it:

• Select "Open" from the "File" menu
• Enter the name of the BN file ("apple.hkb"). You can do this by selecting it from the list of BN files (which have the "hkb" extension).

In figure 2, the BN has been opened and the Hugin GUI is currently working in "edit" mode. We need to be in "edit" mode to edit the BN, so if your network window shows the BN in "run" mode, press the "edit" mode tool button. If you opened it in "edit" mode, you do not need to do anything.

Figure 2: The open BN from the first tutorial in "edit" mode.

Copying Nodes

In the influence diagram in figure 1, there are three nodes very similar to those that we already have. In this case, the Hugin GUI allows you to copy a group of nodes and paste them in another area of the network pane. Here is how to do it:

• Create a rectangle selection with the mouse cursor around all three nodes (drag a rectangle by holding down the left mouse button)
• Select "Copy Nodes" from the "Edit" menu in the Hugin GUI window tool bar
• Select "Paste Nodes" from the "Edit" menu in the Hugin GUI window tool bar
• Move the new group of nodes to a spot where there is room for them

The Hugin GUI generates new names and labels for the new nodes. You can keep the names and change the labels to Sick', Dry', and Loses' (you cannot use "Sick'" as the name because it contains the prime character which is illegal in names). Do you remember how to change the labels?:

• Select the node with the mouse cursor
• Enter "Node Properties" by pressing the node properties tool (the left most tool button in the tool bar of the network window)
• Change the "Label" field
• Press the "OK" button

You perform the steps above for all three new nodes. Your network should then look as the one in figure 3.

Figure 3: The BN extended with Sick', Dry', and Loses'

The next step is to add causal arrows from Sick to Sick' and from Dry to Dry'. If you do not remember how to do this, these steps will help you create one from Sick to Sick':

• Press the causal arrow tool
• Drag an arrow from Sick to Sick' with the left mouse button (while holding down the SHIFT key)

Holding down the SHIFT key enables you to create more causal arrows sequentially without having to press the causal arrow tool again and again.

Adding a Utility Node

So far, the network we have constructed is still a BN. Now, we shall make the first change making it an influence diagram. This change is the addition of a utility node. The utility node we shall add is the Harv node (see figure 1) representing the utility gained from the harvest. Here is how to add it:

• Press the utility tool (to the right of the causal arrow tool)
• Click somewhere in the network pane (a good place would be in the lower right corner besides the Loses' node
• Change the name and label of the new utility node to "Harv"

The harvest depends on the state of Sick' and thus there is an arrow from Sick' to Harv. Add this arrow:

• Press the causal arrow tool
• Drag an arrow from Sick' to Harv

The utility of the harvest was specified to that found in table 1.

You enter the values of table 1 into the utility table of Harv as follows:

• Choose the Harv node as the currently active node by selecting it from the drop down list below the tool bar or simply by double clicking it
• Enter the values from table 1 in the utility table in the node edit pane

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