This paper presents an approach to the evaluation of multimodal dialogue systems, applying usability metrics defined in ISO standards. Users' perceptions of effectiveness, efficiency and satisfaction were correlated with various performance metrics derived from system logfiles and reference annotations. Usability experts rated questions from a preliminary 110-items questionnaire, and an assessment of their agreement on usability concepts has led to a selection of eight main factors: task completion and quality, robustness, learnability, flexibility, likeability, ease of use and usefulness (value) of an application. Based on these factors, an internally consistent and reliable questionnaire with 32 items (Cronbach's alpha of 0.87) was produced. This questionnaire was used to evaluate the Virtual Negotiation Coaching system for metacognitive skills training in a multi-issue bargaining setting. The observed correlations between usability perception and derived performance metrics suggest that the overall system usability is determined by the quality of agreements reached, by the robustness and flexibility of the interaction, and by the quality of system responses.
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