1. A computer-based method of performing an action in a spoken dialog system comprising: retrieving a policy mapping belief states of user intents to actions; receiving a spoken utterance from a user; determining a belief state of the user's intent based on the spoken utterance using a probabilistic ontology tree (POT) comprising a plurality of unobserved nodes each representing a domain concept and comprising a probability distribution table including probabilities that the concept represented by the unobserved node takes each of a plurality of values, wherein each unobserved node is associated with a dialog slot to which the spoken utterance is assigned based on the relatedness between the spoken utterance and the domain concept represented by the unobserved node; selecting an action to take based on the determined belief state and the retrieved policy; and performing the selected action.