It is important to correctly identify different types of statistical studies. Broadly speaking there are two different types of studies - observational and experimental. Some of these studies are retrospective (where data are observed or recorded on variables of interest from events that have already occurred). Some studies are prospective (where data are observed as they happen, as in medical studies). Such studies can be longitudinal (where data are studied across a time period) or cross-sectional (where data are studied at a particular point of time).