Longitudinal studies: objectives and ethical considerations

Studies discussed in this issue have in common that subjects are followed-up, the majority of studies with data collection on at least two occasions. However the need for follow-up in terms of the study hypotheses should be assessed carefully, because such studies are costly and more difficult to carry-out and analyse than cross-sectional studies. Studies are classified according to the nature of the explanatory and outcome variables, whether fixed for each subject, a transitional age, or a changing measurement or characteristic. The information that can be obtained is discussed for each combination of explanatory and outcome variables. Time trends are also considered. The different requirements of reference ranges are included, and ethical considerations particular to follow-up studies are briefly described.