1. University of Paderborn; Department of Computer Science, Computer Science Education Group; Fürstenallee 11 33102 Paderborn Germany
2. SAP Research and University of Darmstadt; Department of Computer Science, Telecooperation Group; Bleichstraße 8 64283 Darmstadt Germany
3. Open University of the Netherlands; Centre for Learning Sciences and Technologies; 6401 DL Heerlen the Netherlands