C. Costa, G. Beham, W. Reinhardt, and M. Sillaots. Proceedings of the 2nd SIRTEL'08 Workshop on Social Information Retrieval for Technology Enhanced Learning, 382, Maastricht, The Netherlands, (September 2008)
U. Shardanand, and P. Maes. CHI '95: Proceedings of the SIGCHI conference on Human factors in computing systems, 1, page 210--217. New York, NY, USA, ACM Press/Addison-Wesley Publishing Co., (1995)