C. Costa, J. Silva, and M. Aparício. Proceedings of the 25th annual ACM international conference on Design of communication, page 263–268. New York, NY, USA, ACM, (2007)
A. Geven, R. Sefelin, and M. Tscheligi. Proceedings of the 8th conference on Human-computer interaction with mobile devices and services, page 157--164. Helsinki, Finland, ACM, (2006)