The ecological fallacy is a widely recognised error in the interpretation of statistical data, whereby inferences about the nature of individuals are based solely upon aggregate statistics collected for the group to which those individuals belong. This fallacy assumes that all members of a group exhibit characteristics of the group at large. Stereotypes are one form of ecological fallacy.
Example: A study is done that shows people from Springfield score higher on the SATs, on average, than people from North Haverbrook. Assuming that a randomly selected individual from Springfield scored higher on the SATs than a randomly selected individual from North Haverbrook is an ecological fallacy. Since the SAT scores given in the study were an average, it is indeed possible that the individual from Springfield scored in the bottom ten percent on the SATs and the individual from North Haverbrook just happened to score in the top 10%.
Example 2: Imagine two communities, Chiptown and Pittsville. Within each community there is a typical divide between the rich and poor, the rich living in gated communities on the hills and the poor living adjacent to the industrial districts that pump carcinogens into their backyards. In both communities, the poor people have a cancer incidence that is many times that of the wealthy people. In Chiptown, where the dominant industry is high-tech computer manufacturing, the overall salaries are higher for both rich and poor people, but the carcinogens spewed into the environment are particularly nasty, giving cancer to nearly all those exposed (poor people). Prof. Newbie comes along and decides to examine the risk factors for cancer. He looks up the cancer rates and median incomes of these two towns on the CDC and U.S. Census webpages. He finds, to everyone's surprise, that the cancer incidence is higher in the wealthier community. He concludes (erroneously!) that higher income is a risk factor for cancer. In fact, we know that exactly the opposite is true. In fact, in the wealthier community of Chiptown, being poor is especially dangerous to one's health.
The ecological fallacy is exceptionally common in population research. Look for it!
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