A long-term project (2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012) was started to test the use of symbolic calculators (SCs) in Bavarian grammar schools (Gymnasien) in Germany with students attending grades 10, 11 and 12. The project started with an evaluation in grade 10 (see Weigand in Int J Technol Math Educ 15(1):19-32, 2008) and accompanied the students in grades 11 and 12. The first years of the project illustrated that a classical quantitative pre-post-test design with project and control groups was not an adequate method to prove the increase of student competency in the project group. Therefore, in the last years, we have concentrated increasingly on an evaluation of new competencies which students developed in SC environments: new working styles, new problem-solving strategies and new ways of achieving a better understanding. A competence model has been developed to evaluate students' abilities while working with the SC within the context of the concept of functions. This article demonstrates the way of constructing the (theoretical) competence model while working on different levels of SC use and different levels of the understanding of functions.

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