Net Construction – Part of Quick Tour

Adding Nodes:  Choose File → New → Network from the menu to create a new belief or decision net window.  To add a nature node (i.e. a “chance node” or “deterministic node”), move the cursor to the toolbar and click the tool button.  When you return the cursor to the new window, it will change to an ellipse, and when you click in the window, a node will be added at the cursor.  The node will be selected when first added (i.e. displayed with negative colors), so if you just press the enter key, the node dialog box will appear which allows you to enter the name, states, etc. of the node.

Utility nodes (also known as “value nodes”) or decision nodes may be added in the same way as nature nodes, by using the or tool buttons respectively.  More Info

Adding Links:   Links may be added by clicking the tool button, then clicking on the node you want the link to come from (i.e. the “parent” node), and finally clicking on the node you want it to go to (i.e. the “child” node).  Alternately, you can click down in the parent node, and while holding the mouse button down, drag the cursor to the child node, and then release it.  If you double-click on a toolbar button then it will create a cursor that you can use to add several nodes or links (without it switching back to the pointer each time).

Another way to add nodes or links is to right-click on the background and choose Modify → New Node or right-click on a node and choose Links.  More Info

Selecting Nodes & Links: To select a node or link, click once on it with the regular pointer cursor, and it will become hilited.  A group of nodes may be selected by clicking down on the background within the window, and then moving the mouse with the button depressed (i.e. “dragging”), so that the selection rectangle is over them.  You can add to or remove from a group of selected nodes by holding down the ctrl key while you select the new nodes.  To delete nodes or links, select them and then press the delete key.  More Info

Undoing Operations:  After doing any operation, you can undo it with Edit → Undo (or pressing ctrl+z).  By repeating this you can undo operations previous to that one (at least 4 operations, and sometimes more if they don't  take much memory).  After undoing one or more operations you can redo them one-by-one with Edit → Redo (or pressing ctrl+shift+z). When exploring with Netica, it is very useful to be able to try a few operations, and then easily undo them.  More Info

Moving Nodes:  A node can be moved by clicking down on it, dragging it to its new position, and then releasing the mouse button.  To move a group of nodes, first select them, and then click down on one of the selected nodes and drag it to its new position.  To change the shape of a link, first select it by clicking on it, then click down on it again and drag the cursor to the point where you want the link bend to be.  More Info

Disconnecting & Reconnecting Links:  A link can be disconnected by clicking on it to select it, then clicking down on the hilited square that forms at its non-arrow end, and dragging it away from the parent node (or by selecting the link and choosing Modify → Disconnect Links).  To reconnect the link to a new node, drag the non-arrow end over the new parent and release the mouse button, or choose  Modify → Reconnect Links.   Disconnection/reconnection is useful to change the links of a net without losing the conditional probability tables of the nodes.  Remember that to just delete a link you simply select it and then press the delete key.  NOTE: If you delete a link and then re-add it, you will have lost the information in the previous CPT; thus if you want to retain the CPTs, use the disconnection command.  More Info

Cutting & Pasting Nodes:  You can cut and paste nodes and subnets within a window, or between windows.  Try opening a net such as Car_Diagnosis_0, select part of it, press ctrl+c (to copy it to the clipboard), ctrl+n (to create a new net), click in the middle of the new window (to indicate where to put it), and then ctrl+v (to paste it into the new net).  You can cut and paste other parts of the net (or other nets), add nodes, etc. and then connect up the disconnected links as described in the previous paragraph. In this way you can take knowledge from previous applications, perhaps save it in a net fragment library, and re-use it to construct a new net for a new application.  More Info

Saving Nets:   At any point you can choose File → Save from the menu to save the current version of the net to file, overwriting the previous one, or File → Save As to save it to a new file.

Node Properties:  You can change the properties of a node by using a node dialog box, which is obtained by double-clicking on the node,( or by right-clicking and choosing Properties).  When you make changes in the dialog box, they won’t actually be applied to the node until you click the “Apply” or “Okay” buttons.  You can change a node’s name or title in the obvious way by typing in the appropriate text field of the dialog box.  To change the name of one of the node’s states, choose the state using the down-arrow beside the “State:” label, and then type in the neighboring text field.  You can add or delete states with the “New” and “Delete” buttons.  Most people find it more convenient to enter or change state names using the text entry area at the bottom of the dialog box.   NOTE:  If you add or delete a state from the node dialog box, you will lose the information in state comments and in the CPT.  Instead, add or delete a state using the right-click menu.  More Info

The text entry area at the bottom of the dialog box can be used to enter or change several different things; you choose the thing to work on with the selector directly above it.  For instance, to enter some text documenting the node, you choose “Description” with the selector, and then type in the text entry box.  Some choices (such as “When Changed”) can’t be modified; they are for viewing only. More Info

Node CPTs:  In order to change the probabilities of a node conditioned on the values of its parents (CPTs), first open the node’s table dialog box by selecting it, and then choosing Table → View/Edit.  You can click on a number (or state name if it’s a deterministic node) to change it.  If no probabilities have yet been entered for this node, you can click on an empty cell to enter a number.  The numbers must be entered as percentages.  You can select some cells by clicking down in a cell that is not currently being edited, and dragging to enclose the cells before releasing the mouse button.  You can select whole rows at a time by clicking and dragging slightly to the right of the double vertical line (this is how you must select deterministic cells, since clicking directly on them brings up the state menu).  There are several items in the Table menu that you can use to set the values of selected cells (such as Uniform Probabilities, Fill Missing, Normalize and Randomize).  Any changes you make in the dialog box will not actually be transferred to the node until you press the Apply or Okay button.  These two buttons do the same thing, but the Okay button also removes the dialog box.  More Info

Node Styles:  To change the display style of some nodes, first select them and then make a choice from the Style menu.  For the Chest Clinic net the most useful displays are Belief-bar or Meter, although if you were creating a net for an end-user you may want to give a node like TbOrCa a style of Hidden.  If you want to hide all the links, you can use Style → Links → Hide Links. The Style → Font choice may be used to change the font and size of the nodes.  More Info