Reproducibility of diffraction experiments provides an indication of data consistency with respect to different variables. The level of reproducibility and accuracy of the diffraction measurements is increased when using calibrated instruments. The paper presents most significant results of the "Calibration Monitoring of Diffractometers", a project using a systematic approach to the Round Robin tests. At the present stage, the results suggest that instrumental performances and other related experimental effects in a diffraction the experiment can be summarised through "characteristic curves".