The E-AT model assumes that each group member has an equal probability to be a representative (see Eqn. (2)). In this section, we argue that group members have different impacts in group item selection decisions. Hence, we introduce the notion of personal impact parameter to model this difference. Accordingly, we propose a personal impact topic (PIT) model by incorporating the personal impacts to control the generation of group representatives. We also provide the model learning algorithm to infer personal impacts, topic distribution of users and item distributions of topics. Finally, to over-come the over-fitting problem when input dataset is sparse, we extend the PIT model by exploring additional social features.