Ecologic study: The analysis is at the level of an entire population rather than at the level of individual persons. The presence of a suspected risk factor can be measured in different populations and compared with the incidence of a paicular disease. Another name for this type of investigation is correlation study, because it seeks to determine the extent to which two characteristics (risk factor and disease occurrence) are related. Cross-sectional studies analyze data collected on a group of subjects at one time rather than over a period of time. Studies with interventions are called experimental or clinical trials. Ref: Greenberg R.S., Daniels S.R., Flanders W.D., Eley J.W., Boring, III J.R. (2005). Chapter 3. Patterns of Occurrence. In R.S. Greenberg, S.R. Daniels, W.D. Flanders, J.W. Eley, J.R. Boring, III (Eds), Medical Epidemiology, 4e.