After having passed a quality control procedure at the local site by a study nurse, all data are transmitted to the data centre by paper (clinical data) or by electronic transfer. The quality and consistency of the data are checked again by qualified collaborators and by computer programs, and are entered into the central database. Each time a follow-up visit has been registered, a graphical summary of the most relevant clinical data is sent to the physician. This feedback is a supplementary quality control and motivates the physician in reporting data of good quality.

At least once a year, a collaborator of the data centre visits each of the 7 collaborating centres. The quality of the information collected at the follow-up visits is checked on a random sample of documents. The collaborator of the data centre extracts the data from the patient chart and compares the result with the data previously reported.