5. A method for performing probabilistic inference in a computer system, the computer system having a processor and a storage device connected to the processor, wherein the storage device stores computer executable instructions and a data structure, the data structure storing a belief network, the belief network probabilistically relating a plurality of different input variables to a plurality of different output decisions, wherein the belief network comprises nodes and arcs indicating relationships between the nodes, a first of the nodes having a decision graph with leaf vertices containing probabilities, one of the leaf vertices being an equivalence vertex such that the decision graph has a plurality of paths to the equivalence vertex, comprising the steps, implemented by the processor through execution of the instructions, of: receiving an observation for a second of the nodes of the belief network; and accessing the decision graph of the first of the nodes to perform probabilistic inference, for a given one of the input variables, using the probability in the equivalence vertex of the decision graph in the first node and the observation so as to select one of the output decisions; and providing the selected one decision as an output of the system.
5. A method for performing probabilistic inference in a computer system, the computer system having a processor and a storage device connected to the processor, wherein the storage device stores computer executable instructions and a data structure, the data structure storing a belief network, the belief network probabilistically relating a plurality of different input variables to a plurality of different output decisions, wherein the belief network comprises nodes and arcs indicating relationships between the nodes, a first of the nodes having a decision graph with leaf vertices containing probabilities, one of the leaf vertices being an equivalence vertex such that the decision graph has a plurality of paths to the equivalence vertex, comprising the steps, implemented by the processor through execution of the instructions, of: receiving an observation for a second of the nodes of the belief network; and accessing the decision graph of the first of the nodes to perform probabilistic inference, for a given one of the input variables, using the probability in the equivalence vertex of the decision graph in the first node and the observation so as to select one of the output decisions; and providing the selected one decision as an output of the system. 6. The method of claim 5 wherein the receiving step includes receiving a characteristic of a user and wherein the accessing step accesses the decision graph of the first node to determine whether the user would likely like to visit a category of web sites based on the characteristic.