Etymology:

The Texas sharpshooter is a fabled marksman who fires his gun randomly at the side of a barn, then paints a bullseye around the spot where the most bullet holes cluster. The story of this Lone Star state shooter has given its name to a fallacy apparently first described in the field of epidemiology, which studies how cases of disease cluster in a population.

Example:

The number of cases of disease D in city C is greater than would be expected by chance. City C has a factory which has released amounts of chemical agent A into the environment. Therefore, agent A causes disease D.

Exposition:

This fallacy occurs when someone jumps to the conclusion that a cluster in some data must be the result of a cause, usually one that it is clustered around. There are two reasons why this is fallacious:

1. The cluster may well be the result of chance, in which case it was not caused by anything.
2. Even if the cluster is not the result of chance, there are other possible reasons for the clustering, other than the cause chosen. For instance, in the Example, if disease D is contagious, it may be clustering around some person who carried it into the city.

At best, the occurrence of a cluster in the data is the basis not for a causal conclusion, but for the formation of a causal hypothesis which needs to be tested. Patterns in data can be useful for forming hypotheses, but they are not themselves sufficient evidence of a causal connection. In short, correlation is not causation.

Exposure:

This fallacy lives up to its striking name because the Texas sharpshooter takes a random cluster, and by drawing a target onto it makes it appear to be causally determined, as if the Texan were shooting at the target. Similarly, when looking at data, there is a danger of jumping to a conclusion that a random cluster is a causal pattern. Without further testing, such a conclusion is seldom if ever justified.

Sources:

• Carroll, Robert Todd, "Texas-Sharpshooter Fallacy", The Skeptic's Dictionary
• Gawande, Atul, "The Cancer-Cluster Myth", The New Yorker, 2/8/1999, pp. 34-37.

Acknowledgment:

The illustration is a detail from René Magritte's painting "Les Charmes du Paysage".