Probabilistic Inference – Part of Quick Tour
Tutorial: To begin, we will read a = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_Bayes_net.htm');return false;">Bayes net stored on disk. Choose File → Open from the menu, and when the standard file-opening dialog box appears, use it to open the file called “Chest Clinic” in the = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_examples_folder.htm');return false;">Examples folder. A window will appear containing a Bayes net consisting of several connected = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_node.htm');return false;">nodes. This very simple net for diagnosing patients arriving at a clinic is a classic example often used to introduce Bayes nets.
You can prepare the net for = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_probabilistic_inference.htm');return false;">inference by choosing Network → Compile from the menu, or by clicking the 
toolbar button (if they are gray, the net has already been compiled). When the compilation is complete, the default display style changes so that nodes are displayed with bar graphs showing the = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_belief.htm');return false;">beliefs for each of their states. You can enter a = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_finding.htm');return false;">finding (also known as an “observation”, or “evidence”) for a node by clicking on the name of the finding to the left of the bar graph. You can also enter a finding by = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_right_click.htm');return false;">right-clicking on the node and choosing Enter Finding.
When the finding is entered, the displays of all the nodes will be adjusted to account for it. For instance, putting an ‘abnormal’ finding for the ‘XRay Result’ node increases the belief that the patient has lung cancer from 5.5% to 48.9%, but then indicating that the patient has made a visit to Asia decreases that belief to 37.1%, because the abnormal XRay is partially = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_explaining_away.htm');return false;">explained away by a greater chance of Tuberculosis (which the patient could catch in Asia).
If you choose “Unknown” from the = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_findings_menu.htm');return false;">findings menu, then any finding for that node will be retracted, and if you choose “Likelihood” you will be queried for the probabilities of an = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_likelihood_finding.htm');return false;">uncertain finding (“virtual evidence”) for the node. You can also click directly on the name of the finding a second time to retract the finding.
Each time you = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_enter_finding.htm');return false;">enter or = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_retract_finding.htm');return false;">retract a finding, the beliefs of all the nodes will immediately be = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_belief_updating.htm');return false;">updated to account for the new information. If you wish updating to only occur when you do a Network → Update command, you can turn off the auto-update feature by = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_toggle_menu.htm');return false;">toggling Network → Automatic Updating.
If you want to observe or change the = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_conditional_probability.htm');return false;">conditional probabilities of a node (which express its = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_node_relation.htm');return false;">relation with its = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_parent_node.htm');return false;">parent nodes), = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_select_node.htm');return false;">select the node by clicking once on it, and then choose Table → View/Edit, or click on the toolbar button with the = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_relation_symbol.htm');return false;">relation symbol: 
. A special window called the = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_table_dialog_box.htm');return false;">table dialog box will open which displays the probabilities in a table. The left-hand side of the = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_contingency_table.htm');return false;">table contains a vertical list of parent configurations, and for each configuration the right-hand side has a few probabilities, expressed as percentages.
Each column of the right-hand side corresponds to a different = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_state.htm');return false;">state of the node. So each number represents the conditional probability that the node takes on the state indicated by the column the number is in, given that the parents have the configuration indicated by the row the number is in. If the node is = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_deterministic_node.htm');return false;">deterministic (e.g. the “TbOrCa” node), then the table dialog box will display a = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_function_table.htm');return false;">function table. This is the same as the conditional probability table, except that the right-hand side simply has the state of the node which is the function value for that parent configuration.
To work with examples of more complex Bayes nets, open the file called “Alarm” (also in the medical domain), or the file called “HailFinder” (weather prediction). You can compile them and do inference in the same way as you did with Chest Clinic. With this example, you can click directly on the name of the finding a second time to retract the finding or hold down the shift key while clicking on the name of the finding to enter a = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_negative_finding.htm');return false;">negative finding.
If you have entered a number of findings for a particular = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_case.htm');return false;">case and wish to save them in their own file, choose Cases → Save Case As and enter the file name in the dialog box which appears. If you want to work on a new case choose Cases → Remove Findings, and then enter the new findings. To recover the original case, choose Cases → Get Case and pick its name from the dialog box which appears.
When you are done with a net window, you can get rid of it by clicking the 
button in its title bar, or by making it the = 4 && typeof(BSPSPopupOnMouseOver) == 'function') BSPSPopupOnMouseOver(event);" class="BSSCPopup" onclick="BSSCPopup('X_PU_active_window.htm');return false;">active window and then choosing File → Close.