  Conversational recommender systems have attracted immense attention recently.
The most recent approaches rely on neural models trained on recorded dialogs
between humans, implementing an end-to-end learning process. These systems are
commonly designed to generate responses given the user's utterances in natural
language. One main challenge is that these generated responses both have to be
appropriate for the given dialog context and must be grammatically and
semantically correct. An alternative to such generation-based approaches is to
retrieve responses from pre-recorded dialog data and to adapt them if needed.
Such retrieval-based approaches were successfully explored in the context of
general conversational systems, but have received limited attention in recent
years for CRS. In this work, we re-assess the potential of such approaches and
design and evaluate a novel technique for response retrieval and ranking. A
user study (N=90) revealed that the responses by our system were on average of
higher quality than those of two recent generation-based systems. We
furthermore found that the quality ranking of the two generation-based
approaches is not aligned with the results from the literature, which points to
open methodological questions. Overall, our research underlines that
retrieval-based approaches should be considered an alternative or complement to
language generation approaches.
