Among educators and researchers there is a wide consensus that flexibility in mental calculation is a key topic of elementary mathematics education. Furthermore, the research interest in students’ individual competencies of flexibility in mental calculation increased in the last decades. Our study focuses on flexibility in mental calculation in elementary students and examines the connection between the cognitive elements which sustain the solution process and the solution tools. Therefore, we developed a semi-structured guided interview and conducted 155 interviews with German third graders. In this paper, we focus on a detailed description of our approach for data collection and the coding of the interviews. Finally, we report descriptive results of the first 20 double-coded interviews of the main study. These results reveal that students justify about 60 percent of the addition and subtraction problems by problem characteristics, number patterns, and relationships.