1. Faculty of Computer Science, Engineering and Economics, Østfold University College, Halden, Norway
2. Computer Science Department, Research Group of Interaction and eLearning (GRIAL), Research Institute for Educational Sciences, University of Salamanca, Salamanca, Spain
3. Escuela Técnica Superior de Ingenieros Informáticos, Universidad Politécnica de Madrid, Madrid, Spain
4. Department of Informatics, University of Oslo, Oslo, Norway