B. Regnell, K. Kimbler, and A. Wesslen. Requirements Engineering, 1995., Proceedings of the Second IEEE International Symposium on, page 40- 47. (1995)
I. Minkkinen, and A. Eerola. SE'07: Proceedings of the 25th conference on IASTED International Multi-Conference, page 93--98. Anaheim, CA, USA, ACTA Press, (2007)