People are not Populations

Ecological Fallacy

Evidence-based medicine uses the statistics of groups and populations as a guide to treating patients. Thus, for example, a supposedly authoritative clinical trial might claim that aspirin reduces the risk of cancer by 25%, although in reality the reduction was only 1 in 1000 (e.g. 4 people in 1000 control subjects got cancer, compared to 3 in 1000 treated subjects). If other clinical trials and maybe a meta-analysis confirm this finding, EBM practitioners consider it scientifically “proven”. People might then be recommended to take aspirin to prevent cancer.

One problem with this idea is the ecological fallacy, which happens when people try to apply group statistics to individuals. To take an example, the average dress size for women in the United Kingdom is 16. But husbands and boyfriends should beware of buying this size clothing as a birthday present for their partner. They might be lucky. But, more likely, the dress will be either too small, “So you think I should be thinner!” or too large, “So you think I’m fat!” Either way, the result will not be helpful.

EBM applies this logical fallacy to patients when it recommends treatments based on large-scale studies. For this reason, you should never assume the results of a clinical trial or media report apply directly to you. Eating cholesterol-laden eggs may, on average, increase the incidence of heart disease slightly in a large population but that is irrelevant to any particular person. You are an individual and can disregard aggregate statistics, in the same way that you would be unlikely to buy average-sized clothes or shoes.

Robinson W.S. (1950) Ecological Correlations and the Behavior of Individuals,  American Sociological Review, 15(3), 351–357.


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