User-centered evaluation of recommender systems with comparison between short and long profile

The growth of the social web poses new challenges and opportunities for recommender systems. The goal of Recommender Systems (RSs) is to filter information from a large data set and to recommend to users only the items that are most likely to interest and/or appeal to them. The quality of a RS is typically defined in terms of different attributes, the principal ones being relevance, novelty, serendipity and global satisfaction. Most existing works evaluate quality of Recommender Systems in terms of statistical factors that are algorithmically measured. This paper aims to explore whether (i) algorithmic measures of RS quality are in accordance with user-based measure and (ii) the user perceived quality of a RS is affected by the number of movies rated by the user. For this purpose we designed, developed and tested a web recommender system, TheBestMovie4You (http://www.moviers.it), which allows us to collect questionnaires about the quality of recommendations. We made a questionnaire and gave it to 240 subjects and we wanted to have as wide a set of users as possible using social web. In a experiment we asked the users to choose five movies (short profile), in a second to choose ten (long profile). Our results show that statistical properties fail in fully describing the quality of algorithms, because with user-centered metrics we can outline an algorithm's features that otherwise could not be detected. The comparison between the two phases highlighted a difference only in three cases out of twenty