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This page was translated from the German original, partly by machine. Some passages may read awkwardly or contain inaccuracies. When in doubt, please read the original.

Base Rate Fallacy

In short

Base probabilities are ignored in favor of striking details.

But he doesn't seem like the type who would do that!

Definition​

The base rate fallacy is the tendency, when estimating probabilities, to ignore the base probability (base rate) and instead to focus exclusively on specific, striking information.

People overestimate the importance of individual details and underestimate statistical foundations, which leads to systematically wrong judgments.

DE: Basisratenfehler

The base rate fallacy is closely connected to several other biases:

  • Representativeness heuristic: Specific details are seen as more typical than they statistically are.
  • Availability heuristic: Striking individual cases are more easily recalled than abstract statistics.
  • Anchoring effect: Initial specific information overshadows the base probabilities.
  • Stereotyping: Outward features are overvalued, statistical realities undervalued.
  • Survivorship bias: Only the visible cases are perceived, the invisible whole population ignored.
  • Conjunction fallacy: Specific combinations seem more likely than their individual components.

Examples​

The Shy Librarian​

A person is described as shy, tidy, and detail-obsessed. Which is more likely: that they are a librarian or a farmer?

Many guess librarian — but ignore that there are far more farmers than librarians. The base rate makes farmer statistically more likely, even if the description fits the stereotype "librarian."

Linda the Bank Teller​

Linda is 31 years old, single, outspoken, and very intelligent. She studied philosophy. As a student, she was deeply concerned with discrimination and social justice, and took part in anti-nuclear demonstrations.

Which is more likely?

  • (A) Linda works as a bank teller
  • (B) Linda works as a bank teller and is active in the women's rights movement

Most people choose (B), even though that is logically impossible: B can never be more likely than A, since B is a subset of A. The detailed description makes us ignore the base rate: there are far more bank tellers than feminist bank tellers.

The Linda problem by Kahneman and Tversky really is THE classic example of the base rate fallacy.

Medical Diagnostics​

A rare test is 99% accurate for a particular disease that affects only 0.1% of the population (one in a thousand). With a positive test, many people think the probability of the disease is 99%.

In fact it is only about 9%. Out of 1,000 people, only 1 person has the disease (true positive), but 10 healthy people get falsely positive test results.

The Terrorist at the Airport​

A security alarm goes off for 99% of all terrorists but has a false-positive rate of 1%. If among one million passengers there are only 10 terrorists:

  • True hits: 10 terrorists × 99% = 9.9 ≈ 10
  • False alarms: 999,990 × 1% = 9,999
  • Given an alarm: only a 0.1% probability of a real terrorist!

The Successful Entrepreneur​

Someone wears a suit, drives an expensive car, and seems self-confident. Are they more likely a successful entrepreneur or an employee?

The base rate says: there are far more well-dressed employees than successful entrepreneurs. Despite the fitting description, "employee" is statistically more likely.

Effects​

  • Systematically wrong risk assessments in medicine
  • Discrimination based on superficial features
  • Poor investment decisions through overvaluing individual cases
  • Inefficient security measures with too many false alarms
  • Prejudices reinforce themselves through ignoring statistical realities

Counter-Strategies​

  • Always ask for base rates: how common is the phenomenon in general?
  • Use absolute instead of relative numbers: 1 in 10,000 instead of 0.01%
  • Natural frequencies: think in groups (100 people instead of percentages)
  • Multiply the probabilities: P(disease|test) ≠ P(test|disease)
  • Be skeptical of "perfect" descriptions: the more specific, the rarer

Sources​