B. Sigurbjörnsson, and R. van Zwol. WWW '08: Proceeding of the 17th International Conference on World Wide Web, page 327--336. New York, NY, USA, ACM, (2008)
L. von Ahn, M. Kedia, and M. Blum. CHI '06: Proceedings of the SIGCHI conference on Human Factors in computing systems, page 75--78. New York, NY, USA, ACM Press, (2006)