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.