Renate Nitsch develops a diagnostic tool that captures typical learning difficulties related to transitions between linear and quadratic functions. In designing the diagnostic instrument, she references documented learning difficulties in the literature, particularly misconceptions, using activity theory as the theoretical foundation for describing learning difficulties and their causes. In the main study, the author identifies nine typical error patterns and demonstrates that these can often be traced back to underlying misconceptions. With the digitization of the test and the introduction of automatic feedback for teachers and students, a flexible testing instrument for everyday school use is ultimately created.