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Cognitive Biases

Texas Sharpshooter Fallacy (Multiple Comparisons Bias)

A cognitive bias where a pattern is identified in data after the fact by focusing on a subset of data that supports a hypothesis, while ignoring the larger context where no pattern exists.

THE IDEA

The Texas Sharpshooter Fallacy, also known as the multiple comparisons bias, occurs when conclusions are drawn from data that have been selectively chosen to confirm a hypothesis. The name comes from a shooter who fires bullets at a barn wall and then draws a target around the tightest cluster of holes, claiming a bullseye. This fallacy is common in statistical analysis, where researchers may test many hypotheses or look at many subgroups without correcting for multiple comparisons, leading to false positives. The classic example is a confidence trickster who sends different stock predictions to a large group, then only continues correspondence with those who received correct predictions. After several rounds, a small subset believes the predictions are accurate and invest money, even though the predictions were random. The key is that the sample is narrowed down to a group where chance alone produced the pattern, but the victims do not see the many incorrect predictions that were discarded.

When to use it

  • Evaluating investment newsletters or stock market gurus who flaunt past correct predictions.

  • Assessing clinical trials that report positive results after many sub-group analyses without correction.

  • Critiquing conspiracy theories that connect disparate events after the fact, ignoring the vast majority of events that do not fit.

  • Understanding why some people believe in psychic abilities after seeing a few correct predictions out of many attempts.

FROM THEORY TO EVERYDAY LIFE

In practice

01

The Mysterious Letter Con

A con artist sends bullish letters to half of 10,000 people and bearish letters to the rest. After the outcome, the artist keeps only recipients of the correct prediction (5,000 people). After further rounds, the artist selects a final pitch list of 500 recipients. Those 500 have seen only successful predictions and are not shown the discarded recipients.

02

Sports Betting ‘Experts’

A sports betting website sends out conflicting predictions to a large email list. After a week, they only continue contacting those who received the correct pick. Over several weeks, a small group sees a nearly perfect record and may pay for premium picks, not realizing that the predictions were random and only survivors are shown.

✳

A MOMENT TO REFLECT

Put it to work.

  1. Can you recall a time when a company or person claimed a special skill based on a series of correct predictions? How might the Texas Sharpshooter Fallacy be at play?

  2. When evaluating scientific studies that report a surprising correlation, what questions should you ask about how many comparisons were made?

  3. If you hear about a stock tip that had an amazing track record over the past few months, what hidden selection bias could be responsible?

CONTINUE THE CONVERSATION

Source texts

  • Fooled by Randomness The Hidden Role of Chance in the Markets and in Life (Nassim Nicholas Taleb)
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