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What is A 10 year long study concerning the use of fluorides and pit and fissure sealants in control of caries on same population group is an example of: and explain in detail?
In the above question, no cause and effect are established before the study. It is longitudinal and experimental trial. A form of cohort study.        
Case-control study usually evaluates from effect to cause backwards. Here the effect is established before study and we search for the cause. It is an example of retrospective study.
Retrospective study is one in which investigation is carried out based on data already collected.
Cohort or prospective study is one in which planning of the entire study is completed before data is collected and analysed.        
In cross-sectional studies, a particular age group of different individuals is investigated only once. In longitudinal studies, a particular individual is studied at different ages.