A prospective 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.