Natural history of disease can be studied from

Longitudinal studies There is an increasing emphasis on the value of longitudinal studies in which observations are repeated in the same population over a prolonged period of time by means of follow-up examinations. Cross-sectional studieshave been likened to a photograph, and longitudinal studies to a cine film. Longitudinal studies are useful (i) to study the natural history of disease and its future outcome {ii) for identifying risk factors of disease, (iii) for finding out incidence rate or rate of occurrence of new cases of disease in the community. Longitudinal studies provide valuable information which the cross-sectional studies may not provide, but longitudinal studies are difficult to organize and more time-consuming than cross-sectional studies. Measurement can also be extended to health states and events. For example, the study of blood pressure levels in a population will reveal the normal values, rather than abnormal ones related to disease. Ref : Park 23rd edition Pgno : 69