Difficulties university students find in learning quantum mechanics (QM) are primarily due to the counterintuitive nature of quantum concepts and to the new and highly mathematical structure in which they are shrouded. Within a research on student understanding of QM focused on the connection between concepts and formal entities representing them, a 21 item questionnaire and follow-up interview protocol were designed. In order to test data gathering instruments and get preliminary results, a calibration study was conducted on a small group of university physics students. Data analysis shows that issues on different topics such as measurement, physical meaning of phase relations, stationarity and time evolution elicited on tested students are often related to the way in which incompatibility is described by quantum state formalism and operator structure of observables.
