Systems thinking is one of the skills necessary for sustainable behavior, especially regarding sustainable consumption. Students are faced with complexity and uncertainty while taking part in it and other daily life aspects. There is a need to foster their competence in this field. From a classroom point of view, the mystery method is an example for implementing education for sustainable consumption and working with complex and uncertain content. With the mystery method students construct an influence diagram, which consists of concepts and requires several skills, especially in decision-making. Using these diagrams as a form of assessment is desirable but also very difficult, because of the mentioned complexity and uncertainty that is part of the task itself. The study presented here tackles this problem by creating an expert based reference diagram that has been constructed with the help of educational data mining. The result shows that it is possible to derive such a reference even if parts remain ambiguous due to the inherent complexity. The reference may now be used to assess students’ systems thinking abilities, which will be undertaken in future research. Beside this, the reference can be used as a reflective tool in lessons, so students can compare their own content knowledge and discuss differences to the experts’ reference.