In epidemiology, ecological studies are used to understand the relationship between outcome and exposure at a population level, where 'population' represents a group of individuals with a shared characteristic such as geography, ethnicity, socio-economic status of employment.
Examples of such studies include investigating associations between units of grouped data, such as electoral wards, regions, or even whole countries.
The map of colon cancer mortality rates in the United States was used by the brothers Cedric and Frank C. Garland to propose the hypothesis that solar ultraviolet B (UVB) radiation, through vitamin D production, reduced the risk of cancer (the UVB-vitamin D-cancer hypothesis).
[14] A randomized controlled trial involving Japanese school children found that taking 1000 IU per day vitamin D3 reduced the risk of type A influenza by two-thirds.
[15] Ecological studies are particularly useful for generating hypotheses since they can use existing data sets and rapidly test the hypothesis.