Thursday, February 10, 2011

Fallacies (3) – anecdotal evidence

There is nothing wrong with the informal use of anecdotal evidence (stories and examples) to explain or support a position or to “prove” a point. For example, we all do it frequently in conversation, and reporters do it in news articles. This use of anecdotal evidence is not usually considered a fallacy.

But in a formal report, such as a report of a scientific study, it is a fallacy to use anecdotal evidence if (1) the evidence is questionable or (2) the amount of evidence, however sound in itself, is insufficient. (More detail here.) Of course, the conclusion may be valid notwithstanding.

An example of the fallacy of anecdotal evidence

For example, during the 1940s the U.S. Government decided to introduce sodium fluoride into municipal water systems in the United States. This decision was based on only one study of fluoridation, conducted in the cities of Newburgh and Kingston, New York. In that study, fluoride appeared to help prevent dental cavities in children between five and nine years of age.

Today, many dentists and doctors are reconsidering whether fluoridation was prudent. I am not arguing for or against fluoridation here; my point is that the government apparently began, based on a small amount of evidence, a massive public health program that treated people of all ages. This action was probably a fallacy of anecdotal evidence.

The Takeaway: If you have occasion to write formal reports, be aware of the fallacy of anecdotal evidence. Ask whether your evidence is: (1) sound in itself and (2) sufficient to warrant generalization.

Related: Fallacies (2) – cherry picking

See disclaimer.

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