Cells Containing X
Meaning: A cell of the table dialog box table with just an X in it indicates that no value is required for the cell, because that configuration of parent values will never occur. It actually doesn’t say anything about the relationship at hand, but just that whoever was building the Bayes net didn’t feel it was necessary to enter a value because in the context of the overall net it is not required. If Netica is later doing inference and discovers that configuration of parent values can occur, it will alert you with a message in the = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_Messages_window.htm');return false;">messages window.
Importance: If you put any arbitrary value in such a cell, all inference results will be the same, because the value will not be used. The best value to put in the cell is the true value that would be applicable if the relationship between the parents was different, but if that value doesn’t exist, or you don’t know it, or don’t have time to determine it, it is much better to use the X feature than to just put in an arbitrary value, for three reasons. First, during inference Netica performs an important check for you as to whether the configuration is really impossible. Second, it documents your understanding at the time you were building the Bayes net for anyone else who later works with it. Third, if you later change other parts of the net, or you copy and paste this node into a different net, or put it in a net fragment library, those parent configurations may become possible.
Setting: You can set a cell to X by selecting the cell and then typing X (for a probabilistic table), or by choosing ‘Impossible’ from the cell’s pop-up menu (for a deterministic table). For probabilistic tables, if one cell has an X, then all the cells in that row must have an X (because they all correspond to the same parent configuration).