Abstract
In this paper we address the problem of measuring the overall effectiveness of group formation in virtual communities. Group formation is often driven by the combination of similarity and trust measures which are usually exploited with the recommendations provided by all the community members (global reputation). In this work propose a specific index to measure the effectiveness of group formation, and to exploit the local reputation in place of the global one. The use of local reputation will allow group administrators to save a significant amount of computationally and/or communicational tasks. We designed an algorithm to form effective groups in virtual communities and tested it on real data.