1. A method of forming a decision tree used in speech processing, the method comprising: grouping at least two tokens to form a first possible cluster; determining a mutual information score based on the first possible cluster; grouping at least two tokens to form a second possible cluster; determining a mutual information score based on the second possible cluster; selecting one of the first cluster and the second cluster based on the mutual information scores associated with the first cluster and the second cluster; and using the selected cluster to form a question in the decision tree.