J. Nielsen, and T. Landauer. CHI '93: Proceedings of the INTERACT '93 and CHI '93 conference on Human factors in computing systems, page 206--213. New York, NY, USA, ACM, (1993)
M. Andreasen, H. Nielsen, S. Schrøder, and J. Stage. CHI '07: Proceedings of the SIGCHI conference on Human factors in computing systems, page 1405--1414. New York, NY, USA, ACM, (2007)