It becomes a good habit to organize a data mining cup, a competition or a challenge at machine learning or data mining conferences. The main idea of the Discovery Challenge organized at the European Conferences on Principles and Practice of Knowledge Discovery in Databases since 1999 was to encourage a collaborative research effort rather than a competition between data miners. Different data sets have been used for the Discovery Challenge workshops during the seven years. The paper summarizes our experience gained when organizing and evaluating the Discovery Challenge on the atherosclerosis risk factor data.