Normal ranges, which were set up for quality control, serve to define an acceptable qualitative niveau of treatment. To establish such criteria, it is important to consider group-specific characteristics during investigations. Results of clinical trials are the basics to determine the normal range of parameters of quality control. For external evaluation of data, which were not obtained in clinical studies, but in daily routine, normal ranges of the regional hospitals, which participated in the trial, are recommended to be included. However, an adequate total number of data is the prerequisite to allow statistically relevant evaluation.