Entering observations (evidence) is one of the basic operations on a probabilistic model. It amounts to adjusting the model to a new situation, one in which more information is available. It allows to query the system subsequently about the new, posterior probability distributions. If you have gone through the Building a Bayesian network tutorial introduction, you might remember that you have already entered evidence for the Expert forecast node. Let us go through this again.
You may load the model VentureBN.xdsl created in Building a Bayesian network from the Example Networks folder.
To enter evidence into your model, right-click on the node in question (in the picture below, node Success of the venture) and choose Set Evidence.
Alternatively, select the node in question and choose Set Evidence submenu from the Node Menu.
Yet another way is double-clicking on the observed state in the Bar chart view of the node
To retract evidence, right-click on the node with previously entered evidence (in the picture below, node Success of the venture) and choose Clear evidence.
Alternatively, select the node in question and choose Clear evidence from the Node Menu.
You can also retract all evidence by choosing Clear all evidence from the Network Menu:
To enter different evidence instead of retracting evidence altogether, just set different evidence from the Set Evidence sub-menu.
Entering evidence in continuous/equation nodes invokes a dialog that allows for entering a numerical value:
That value is subsequently displayed at the bottom of the Bar Chart view of the node:
and can be changed by just double-clicking on that value
This allows for fairly quick entering and changing of evidence.