Evaluation of spoken dialogue systems has been traditionally carried out in terms of instrumentally or expert-derived measures (usually called “objective” evaluation) and quality judgments of users who have previously interacted with the system (also called “subjective” evaluation). Different research efforts have been made to extract relationships between these evaluation criteria. In this paper we report empirical results obtained from statistical studies, which were carried out on interactions of real users with our spoken dialogue system. These studies have rarely been exploited in the literature. Our results show that they can indicate important relationships between criteria, which can be used as guidelines for refinement of the systems under evaluation, as well as contributing to the state-of-the-art knowledge about how quantitative aspects of the systems affect the user’s perceptions about them.