GeNIe Tutorials: Tutorial 8 - Evaluating a model
From DSL
Objective of the tutorial: To learn how to update a model.
Estimated Time: 10 M
At the end of the tutorial you will be able to:
- Update a model with the selected inference algorithm.
- Specify target nodes in the model.
- Select from various inference algorithm supported by GeNIe.
Once you design a decision model you need to update the model using an algorithm to evaluate it and see the values of the probabilities of various states of the nodes in the model.
GeNIe operates in two modes: immediate and lazy updating. In lazy updating mode every time you change the evidence of a node, or control a value of the node, then you need to update the model explicitly, to view the result of the changes. In immediate updating mode, GeNIe automatically updates the model as soon as any change is made to the model. Hence you do not need to update the model explicitly.
To switch between the two modes, choose Update immediately from the Network menu.
The lazy updating mode, is useful when the model is in development stage or when it is so large that updating takes an annoyingly long time.
You can explicitly update the model either by choosing Update from the Network menu or alternatively by pressing the 
[ Update ] button.
GeNIe uses status icons for the nodes that can be used to know whether a particular node has been updated or not. If a particular node is not updated then it will have a small question mark [ 
] icon on it.
The values of such nodes cannot be examined as they are not accurate, and GeNIe will not display the Value tab in the Node Properties Sheet for these nodes.
When the values are up to date, GeNIe displays a small tick [ [[Image:UpdateStatusIcon.jpg] ] icon on the node.
In addition, the user can specify which nodes are of particular interest and will be potentially examined by designating them to be targets. GeNIe guarantees that all target nodes will be updated by its reasoning algorithms but does not give any guarantees with respect to other nodes. To set a node to be a target, select it and then choose Set target from the Node menu.
Alternatively, right-click on the chosen node and choose Set target from the its pop-up menu.
A target node will display a small target icon [ 
].
When no nodes are designated as targets, GeNIe treats all the nodes as targets.
GeNIe is primarily a research and teaching tool and implements a variety of algorithms. The user can choose which algorithm should be used for updating by selecting an algorithm from the list displayed in the Network menu.
We would like to stress that choice of algorithms has a useful practical applications: when a networks cannot be updated using the fastest known exact algorithm, the Clustering algorithm (GeNIe's default), the user can still update the model using an approximate algorithm. Tradeoff between precision and computation time can be controlled by selecting the number of samples. For more information on the algorithms supported by GeNIe, see the Inference Algorithms section.
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