(For an environmental definition of this term see Ecology.) An ecological study is an epidemiological study in which the unit of analysis is a population rather than an individual. For instance, an ecological study may look at the association between smoking and lung cancer deaths in different countries. An ecological study is normally regarded as inferior to non-ecological designs such as cohort and case-control studies because it is susceptible to the ecological fallacy. An example of an ecological study is the analysis of the effects of disinfection byproducts on newborn babies, using 109 Massachusetts towns as units of analysis (Wright et al. 2004).
Ecological studies can be easily confused with cohort studies, especially if different cohorts are located in different places. The difference is that in the case of ecological studies there is no information available about the individual members of the populations compared (e.g. comparing several states based on state-wide average air pollution and state-wide average prevalence of respiratory diseases); whereas in a cohort study the data pair exposure/health is known for each individual.
In spite of their weaknesses, ecological studies are useful because they can be carried out easily, quickly and inexpensively using data that are generally already available. If interesting and strong associations are observed, the results of ecological studies can provide the opportunity for later, more carefully designed studies (though more expensive and time-consuming) to build on the initial observations.