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Ecological fallacy is improperly inferring an association (or lack of association) at an individual-level based on a group-level relationship.
Ecologic studies use measures taken at the level of a group (such as a country, school, or hospital) rather than at the individual (such as patient) level. Ecologic studies are widespread in behavioral medicine literature due their low cost and convenience since ecologic data can often be obtained through census records or existing surveys and records. Two typical behavioral medicine ecologic examples are a study investigating the association between alcohol availability and violence (Gorman et al. 2005) in which data were collected at the census tract level and a study investigating the association between needle exchange programs and HIV seroprevalence among injecting drug users (Hurley et al. 1997) in which data were collected at the city level.
When risk factors and outcomes are measured at an aggregate level, the relationship between the group-level variables may be different than the relationship between variables measured at the individual level. An often-cited example used to illustrate the issue involved a nineteenth-century study which found higher suicide rates within Prussian provinces that had higher proportions of Protestant residents (Durkheim 1951). The conclusion that Protestant individuals (rather than Catholic individuals) were more likely to commit suicide cannot be inferred based on the observed association among the provinces (Morgenstern 1982; Robinson 2009). One possible scenario is that Catholic residents within the largely Protestant provinces had the high suicide rates, resulting in a positive association between percent Protestant and suicide rate. Extrapolation of aggregate results to individuals is a mistake in logic which can lead to a potentially misleading conclusion.
Because of the many limitations of ecologic studies, including ecological fallacy, they are often used as exploratory or hypothesis-generating studies rather than as confirmatory.
References and Further Reading
- Durkheim, E. (1951). Suicide: A study in sociology (trans: Spaulding, S.). Glencoe: Free Press. (Original Work Published 1897).Google Scholar