# Ad aptation

## (Tutorial)

Consider the Bayesian belief network (Bbn) shown in figure 1. To learn more about the domain of this Bbn refer to the section network samples.

Figure 1: Bbn Representation of "Chest Clinic"

Experience table or tables are necessary to use adaptation. To add experience table to all discrete chance nodes in the domain click somewhere in the network pane, push the right mouse button and then choose "Add Experience Table to All Discrete Chance Nodes". It also possible to add experience tables to a subset of discrete chance nodes in the domain. To do the later select a specific discrete chance node then right click and select "Add Experience Table".

In our example it is justified to add experience tables to all the nodes in the domain except "Tuberculosis or cancer" since this node is a logical or and no experience can be gained on logical or nodes. To see the created experience table select the desired node and right click then select "Show Experience Table".

As mentioned earlier, the experience table of a node represents the experience count of the parent configurations. For example the experience table for node "Dyspnoea?" which is shown in figure 2,

Has bronchitis | yes | no
Tuberculosis or cancer | yes | no | yes | no
Experience count(s) | 0 | 0 | 0 | 0

Figure 2: Experience Table for the "Dyspnoea?" Discrete Chance Node

Represents the number of observations of different parent configuration. The value zero is an invalid experience count thus the value must be greater than 0 to activate adaptation. If our believe of the correctness of the present conditional distribution probability is high then the experience count must have a high value otherwise the value of the count should be low. In this case we assume that our believe in the correctness of the current conditional distribution is low thus we set the initial experience count to a low number, for instants "10". Figure 3 show the initial experience count table for "Dyspnoea?" discrete chance node.

Has bronchitis | yes | no
Tuberculosis or cancer | yes | no | yes | no
Experience count(s) | 10 | 10 | 10 | 10

Figure 3: The Updated Experience Table for the "Dyspnoea?" Discrete Chance Node

Note that it is not necessary to activate experience count or enter the same experience count for every parent configuration. For instants the initial experience count values can be set to "10,0,100,0". Note that adaptation requires at least a node with experience table otherwise it is not possible to adapt the domain. Now add experience table to the every node in the Bbn (except "Tuberculosis or cancer" node) and set the initial experience counts to 10. The domain is now ready for adaptation.

An adaptation step consists of entering evidence, propagating, and finally updating (adapting) the conditional probability and experience tables.

Lets concentrate on one of the nodes namely "Smoker?” The conditional distribution probability of this node prior to any adoption is S (0.5,0.5). Now enter the following observations:

- Node S is in state 0 ("yes")
- Node X is in state 0 ("yes")
- Node D is in state 0 ("yes")

then propagate the evidence. Next push the adaptation button which is shown in figure 4.

Figure 4: The Adaptation Icon

Keep clicking on the adaptation button a couple of times. Each time the adaptation button is pushed the probability of this observation (i.e. P(All)) increases. Now initialise the Bbn and observe the conditional distribution probability of the "Smoker?” As you can see the conditional distribution probability is no longer S(0.5,0.5). Actually no conditional distribution probability is the same. This indicates that based on the new observations the conditional distribution probabilities has been changed. I.e. if the experience tables are now deleted or the values of the experience tables are set to zero then the current distribution probabilities will be the new conditional distribution probabilities of the nodes.

To extend the adaptation with optional fading we need to add fading table to some or all of the discrete chance nodes in the Bbn. To add fading tables to all discrete chance nodes in the Bbn click somewhere in the network pane, push the right mouse button and then choose "Add Fading Table to All Discrete Chance Nodes". To add fading tables to a discrete chance node in the Bbn select the specific discrete chance node then right click and select "Add Fading Table". Note that if a node does not have an experience table then it is not possible to add fading table to the node.

| HUGIN Expert A/S | , 2000

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