Prospective study: The studies in which individuals are followed-up over the period of time. A general example of this type of investigation is where the samples of individuals are exposed and not exposed to the possible risk factor for a specific disease, are followed forward in time to determine what effects occur to them with respect to the illness under the investigation. At the end of the appropriate time period a comparison of the incidence of the disease between the exposed and non-exposed is made. A classic instance of such a study is that undertaken among the British doctors in the 1950s, to examine the relationship between smoking and death from the lung cancer. All the clinical trials are prospective.