Clinical study designs are formulation of trials and experiments in medical and epidemiological research. frequency of disease (or any other outcome of interest) in relation to the level of exposure in several groups of people (or in the same group over different periods of time) are called ecological studies.
There are interesting examples of Ecological studies:
(1) The study by John Snow regarding a cholera outbreak in London is considered the first ecological study to solve a health issue. He used a map of deaths from cholera to determine that the source of the cholera was a pump on Broad Street. He had the pump handle removed in 1854 and people stopped dying there. It was only when Robert Koch discovered bacteria years later that the mechanism of cholera transmission was understood.Curiously, Snow’s ecological observations are not referred to as an ecological study, at least in a standard text on epidemiology.
(2)A fascinating study (in figure) questions the link between prostate cancer in men and the Pill. David Margel and Neil Fleshner, of University of Toronto in Ontario, looked at contraceptive pill usage and incidence of prostate cancer in 88 countries around the world. In every case, a significant correlation between the two was found. Although studies continue to look at various possibilities, and findings are inconclusive, the scientists consider this a valid and strong component in the mystery of the increase in prostate cancer; the fact is chemicals like estrogen pass into the urine and ultimately make their way into the water supply.
This highlights the importance of ecological studies in finding associations which leads to other types of study designs that conform the association.