HI I'm Phd student, working in Recsys \collaborative filtering. I'm still new in this field. I have query. I know how prediction ratings are evaluated , but I don not know how recommendation lists are evaluated. My work is nor relate with ratings and predicate the ratings but with recommendations list. if I predicate an list for one user, how I know if this list is right or not. where this list has new items for user that he is not know it , buy it, read it, show it, etc. I have problem in understanding this issue. thanks in advance