The source of data should be defined. It is beneficial to work on well defined populations (with respect to area of residence, occupation, place of employement, ..etc.) to be able to control and follow-up study subjects. A standard primary source of information should be used to collect data from all study subjects. Other standard sources could be used to ascertain the quality of collected data. Methods should be developed to motivate subjects to participate in the study and to assure their compliance.