Glossary
The Base Rate Fallacy
What is the base rate fallacy?
The base rate fallacy, also called base rate neglect, occurs when a probability judgment gives too little weight to how common an outcome is in the relevant population.
Example
In the lawyer–engineer experiments summarized by Tversky and Kahneman, personality descriptions could dominate judgments even when the stated proportions of lawyers and engineers changed. Without a description, participants used those proportions. Neglecting base rates was not inevitable.
A numerical base rate example
Imagine a factory where 10 of every 1,000 items are defective. A screening system flags all 10, but also flags 99 good items. Of the 109 flagged items, only 10 are defective: about 9.2%. Ignoring the low base rate of defects can make a flag seem more conclusive than it is. These numbers are hypothetical.
Why it matters
A vivid description is not a substitute for the background numbers. Combine the base rate with the reliability of the evidence about the particular case. Ignoring prevalence can produce an overestimate or an underestimate; the direction depends on the probabilities involved.
The conjunction fallacy concerns a different mistake: judging a combination of events as more probable than one of its components.