A second category system was established for quantifying drawn elements (e.g., base pairs) and, at the same time, for analyzing in which grade it was drawn (e.g., single base pairs or base pairs that are inscribed and linked via intramolecular hydrogen bonds). We iteratively categorized the students' drawings and inscriptions by following the method of inductive category development 34. We obtained six main categories with a total of nine sub categories (Table 1). Each sub category can reveal a different level of representation. To enable an objective analysis, we created additional guidelines for a clear classification (Appendix C).