L. von Ahn, and L. Dabbish. CHI '04: Proceedings of the SIGCHI conference on Human factors in computing systems, page 319--326. New York, NY, USA, ACM Press, (2004)
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)