Texas sharpshooter fallacy
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The Texas sharpshooter fallacy is a logical fallacy in which information that has no relationship is interpreted or manipulated until it appears to have meaning. The name comes from a story about a Texan who fires several shots at the side of a barn, then paints a target centered on the hits and claims to be a sharpshooter.
The fallacy does not apply if one had an ex ante, or prior expectation of the particular relationship in question before examining the data. For example one might, previous to examining the information, have in mind a specific physical mechanism implying the particular relationship. One could then use the information to give support or cast doubt on the presence of that mechanism. Alternatively, if additional information can be generated using the same process as the original information, one can use the original information to construct a hypothesis, and then test the hypothesis on the new data. See hypothesis testing. What one cannot do, is use the same information to construct and test the same hypothesis — to do so would be to commit the Texas sharpshooter fallacy.
The fallacy is related to the clustering illusion, which refers to the tendency in human cognition to interpret patterns in randomness where none actually exist.
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[edit] Examples
- Attempts to find cryptograms in the works of William Shakespeare, which tended to report results only for those passages of Shakespeare for which the proposed decoding algorithm produced an intelligible result. This could be explained as an example of the fallacy because passages which do not match the algorithm have not been accounted for. The fallacy could also be an explanation for cryptograms in the Bible.
- A million people participate in a raffle. The raffle is drawn and Joe wins. However, Jane points out that "the odds of Joe winning are a million to one. There's no way he could've won a random draw. He must have cheated and rigged the raffle." Obviously, the chances of anyone else winning the raffle was also a million to one, and Jane could've accused everyone of cheating. However, the chances of somebody winning is 100% guaranteed. So long as the raffle was drawn, somebody had to win. Joe simply lucked out, and it's not logical for Jane to accuse Joe of cheating based on this reasoning. As with the Texas sharpshooter, Jane draws the bullseye around Joe after the random draw had taken place, making Joe's win seem like a non-random event.
[edit] Related logical fallacies
- "The roulette ball has landed on odd numbers eight times in a row. Therefore, it's more likely to land on an even number next time." (gambler's fallacy.)
- "More children in town A have leukemia than in town B. Therefore, there must be something wrong with town A." (cum hoc ergo propter hoc or post hoc ergo propter hoc.)
