This study examines the criteria questioners use to select the best answers in a social Q&A site (Yahoo! Answers) within the theoretical framework of relevance research. A social Q&A site is a novel environment where people voluntarily ask and answer questions. In Yahoo! Answers, the questioner selects the answer that best satisfies his or her question and leaves comments on it. Under the assumption that the comments reflect the reasons why questioners select particular answers as the best, this study analyzed 2,140 comments collected from Yahoo! Answers during December 2007. The content analysis identified 23 individual relevance criteria in six classes: Content, Cognitive, Utility, Information Sources, Extrinsic, and Socioemotional. A major finding is that the selection criteria used in a social Q&A site have considerable overlap with many relevance criteria uncovered in previous relevance studies, but that the scope of socio-emotional criteria has been expanded to include the social aspect of this environment. Another significant finding is that the relative importance of individual criteria varies according to topic categories. Socioemotional criteria are popular in discussion-oriented categories, content-oriented criteria in topic-oriented categories, and utility criteria in self-help categories. This study generalizes previous relevance studies to a new environment by going beyond an academic setting.