The weakest of all epidemiology studies are ecological ones, observational studies that compare large populations rather than individually track participants who are divided into groups. An example of an ecological study would be a comparison of health statistics in Europe against those in the United States. While the populations are large, the ability to control confounding factors is very limited because there are just too many of them.
It is difficult to draw conclusions about particular individuals using an ecological study. Drawing such conclusions is called an “ecological fallacy.” For example, if a study finds that Europeans have lower heart disease rates, it would be an ecological fallacy to assume a specific European would have a lower risk of heart disease than a random American. “Ecological studies can provide useful clues, but conclusions about individuals are in general only weakly supported by data on groups,” explains the late David Freedman, professor of statistics at the University of California, Berkeley.(1)
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(1) David A. Freedman, “The Ecological Fallacy,” Department of Statistics University of California, Berkeley, August 1, 2002.