This type of observational study uses aggregate level exposures and outcomes. The unit of observation is aggregate (i.e. cities, decades, countries) and no individual level data is available.  They are useful to monitor population health, help with public health planning, and make large scale comparisons. These studies can be cheap and easy to conduct, typically because they use existing data sources but they can suffer from a unique type of bias, called Ecological Fallacy. This fallacy is the assumption that what is seen for the aggregate level holds true for individuals, when in fact it does not.