As spoken dialog systems become more complex, efficient ways to evaluate them in early development stages are required. User simulation has been successfully used for this purpose. While current user models describe behavior on the level of overt be-havior, modeling aspects of cognition can reveal direct insights into usability problems. Thus, in this paper we propose two models related to grounding in dialog: a model of the belief the user has about the system state, and a model of vocabulary alignment. We show that parameters derived from these models are significantly correlated with the users' quality perception.

Index Terms: user model, evaluation, grounding