1. A method for processing a voice command using a statistical dialog model, comprising: determining, in response to receiving the voice command, a belief state as a probability distribution over states organized in a hierarchy with a parent-child relationship of nodes representing the states, such that the belief state includes the hierarchy of state variables defining probabilities of each state to correspond to the voice command, wherein a probability of a state of a child node in the hierarchy is a conditioned on a probability of a state of a corresponding parent node; and selecting a system action based on the belief state, wherein steps of the method are performed by a processor.