Generating post hoc review-based natural language justifications for recommender systems

In this article, we present a framework to build post hoc natural language justifications that supports the suggestions generated by a recommendation algorithm. Our methodology is based on the intuition that reviews’ excerpts contain much relevant information that can be used to justify a recommendation; thus, we propose a black-box explanation strategy that takes as input a recommended item and a set of reviews and builds as output a post hoc natural language justification which is completely independent of the underlying recommendation model. To validate our claims, we also introduce three different implementations of our conceptual framework: the first one uses natural language processing and sentiment analysis techniques to identify relevant and distinguishing aspects discussed in the reviews and combines reviews’ excerpts mentioning these aspects in a natural language justification which is presented to the target user. The second implementation extends the first one by introducing automatic aspect extraction and text summarization, which are exploited to generate a unique synthesis presenting the main characteristics of the item that is used as justification. Finally, the third implementation tackles the problem of generating a context-aware justification, that is to say, a justification that differs on varying of the different contextual situations, by automatically learning a lexicon for each contextual setting and by using such a lexicon to diversify the justifications. In the experimental evaluation, we carried out three user studies in different domains, and the results showed that our methodology is able to make the recommendation process more transparent, engaging and trustful for the users, thus confirming the validity of the intuitions behind this work.