1. Andersen, E., Gulwani, S., Popovic, Z.: A trace-based framework for analyzing and synthesizing educational progressions. In: Proceedings of the SIGCHI Conference on Human Factors in Computing Systems, CHI ’13, pp. 773–782 (2013). ISBN 978-1-4503-1899-0
2. Aponte, M.-V., Levieux, G., Natkin, S.: Measuring the level of difficulty in single player video games. Entertain. Comput. 2(4), 205–213 (2011)
3. Bakkes, S., Whiteson, S., Li, G., Vişniuc, G.V., Charitos, E., Heijne, N., Swellengrebel, A.: Challenge balancing for personalised game spaces. In: 2014 IEEE Games Media Entertainment, pp. 1–8. IEEE (2014)
4. Bartle, R.: Hearts, clubs, diamonds, spades: players who suit muds. J. MUD Res. 1(1), 19 (1996)
5. Bartle, R.: Designing Virtual Worlds. New Riders Games, Indianapolis (2003)