The Texas Sharpshooter Fallacy
Alias: The Texas Sharpshooter Effect*
…[T]he epidemiologist Seymour Grufferman coined the term “Texas sharpshooter effect.” Stand way back and blast the side of a barn with a shotgun and then find some holes that are crowded together. Draw a circle around them and you have what looks like a bull’s-eye.*
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 disease spreads in a population.*
Each year…epidemiologists regularly hear from people worried that their town has been plagued with an unusually large visitation [of cancer cases]. … The Erin Brockovich incident, one of the most famous, is among the many that have been debunked. Hexavalent chromium in the water supply of a small California town was blamed for causing cancer, resulting in a $333 million legal settlement and a movie starring Julia Roberts. But an epidemiological study ultimately showed that the cancer rate was no greater than that of the general population. The rate was actually slightly less.*
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:
- The cluster may well be the result of chance, in which case it was not caused by anything.
- 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, if a disease is contagious, it may be clustered around a carrier.
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.
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.