M. Tedre, and J. Pajunen. Proceedings of the 13th Koli Calling International Conference on Computing Education Research, page 97--104. New York, NY, USA, ACM, (2013)
L. Barker, K. Garvin-Doxas, and E. Roberts. Proceedings of the 36th SIGCSE technical symposium on Computer science education, page 421--425. New York, NY, USA, ACM, (2005)