We describe recommender systems and especially case-based recommender systems. We define a framework in which these systems can be understood. The framework contrasts collaborative with case-based, reactive with proactive, single-shot with conversational, and asking with proposing. Within this framework, we review a selection of papers from the case-based recommender systems literature, covering the development of these systems over the last ten years.

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Abstract. Critiquing is a powerful style of feedback for case-based recommender systems. Instead of providing detailed feature values, users indicate a directional preference for a feature. For example, a user might ask for a 'less expensive' restaurant in a restaurant recommender; 'less expensive' is a critique over the price feature. The value of critiquing is that it is generally applicable over a wide range of domains and it is an effective means of focusing search. To date critiquing approaches have usually been limited to single-feature critiques, and this ultimately limits the degree to which a given critique can eliminate unsuitable cases. In this paper we propose extending the critiquing concept to cater for the possibility of compound critiques-critiques over multiple case features. We describe a technique for automatically generating useful compound critiques and demonstrate how this can significantly improve the performance of a conversational recommender system. We also argue that this generalised form of critiquing offers explanatory benefits by helping the user to better understand the structure of the recommendation space.