D. Sonntag, and M. Möller. Proceedings of the International Conference on Knowledge Management and Information Sharing (KMIS), Madeira, Portugal, (2009)
S. Dumais, E. Cutrell, J. Cadiz, G. Jancke, R. Sarin, and D. Robbins. SIGIR '03: Proceedings of the 26th annual international ACM SIGIR conference on Research and development in informaion retrieval, page 72--79. ACM Press, (July 2003)