study follows a group of people,that is, data is collected
over time. Prospective studies are done when the investigator is
interested in not only present characteristics of a population,
but also with changes in characteristics over time. An example of
a popular longitudinal study is NHANES (the National Health and
Nutrition Examination Survey) in which health and dietary data is
collected every X number of years.
The big drawback
to longitudinal studies is the time it takes to realize the results.
But, although a cross-sectional study is quicker, differences over
time attributed to individual change can be more reliably assessed
in a longitudinal study.