# Base rate neglect

*Also called: Base rate fallacy*

- **Cluster:** Evidence and sampling — The data that was gathered, and the data that was not
- **Page:** https://fallacy.wiki/fallacy/base-rate/
- **In Arabic:** https://fallacy.wiki/ar/fallacy/base-rate.md
- **Licence:** CC BY 4.0 — https://creativecommons.org/licenses/by/4.0/

## What it is

Reading a test result or signal without accounting for how rare the underlying condition is.

## In the wild

> The test is 99 percent accurate and positive, so I almost certainly have it.

## How to answer it

Ask how common the condition is in the population before reading the result.

## Origin and naming

Named 'base rate neglect' by psychologists Amos Tversky and Daniel Kahneman, who showed in the 1970s that people ignore how common a condition actually is once they are handed an individuating detail. The taxi-cab problem became the canonical demonstration. The math behind the correction is Bayes' theorem.

## More places it shows up

- A start-up rejects a candidate flagged by a fraud detector that is 95% accurate, forgetting that only 1 in 500 applicants is actually fraudulent — so most flags are false alarms.
- A clinic panics over a positive screening whose accuracy is 99%, without mentioning the disease appears in about 1 in 10,000 people.
- An airline safety analyst declares an incident pattern 'alarming' after two near-misses in a month, ignoring that the airline runs ten thousand flights a week.
- A hiring platform claims its personality quiz predicts success with 90% accuracy, but high performers are so rare in the applicant pool that the quiz labels mostly false positives.

## How to spot it

- Only one number is quoted — the accuracy of the test. Nobody says how common the thing being detected is.
- The reaction is a shock reaction: 'positive, therefore certain'. Probability of the result never meets probability of the condition.
- Ask 'out of a hundred people like this, how many actually have it?' and the whole case needs redoing.

## Nearby

- [Conjunction fallacy](https://fallacy.wiki/fallacy/conjunction.md)
- [Ecological fallacy](https://fallacy.wiki/fallacy/ecological.md)
- [Survivorship bias](https://fallacy.wiki/fallacy/survivorship.md)

## Read more

- [Wikipedia](https://en.wikipedia.org/wiki/Base_rate_fallacy)
- [The Fallacy Files](https://www.fallacyfiles.org/baserate.html)

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From fallacy.wiki — a bilingual field guide to 91 logical fallacies, published by phi holdings. Licensed CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/): reuse freely with attribution to fallacy.wiki.