Base Rate Fallacy

Nicholas Epley

Imagine that you meet Tom one evening at a party. He is somewhat shy and reserved, is very analytical, and enjoys reading science fiction novels. What is the likelihood that Tom works as a computer scientist? The answer depends on both the knowledge you have about Tom and the number of computer scientists that exist in the population. Tom fits the stereotype of a computer scientist, but there are relatively few computer scientists in the general population compared to all other occupations. The knowledge you have about Tom is often called individuating or case-based information, whereas knowledge about the number of computer scientists in the general population is often called distributional or base rate information. When presented with both pieces of information—be it when judging the risk of contracting a disease, when judging the likelihood of a defendant's guilt, or when predicting the likelihood of future events—people often base their ...

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